Do not remove menu -pluginWillBeUnloadedFromMemory

Fixes a crash that would occur under certain circumstances that is
traced back to this particular snippet of code. As plugins cannot be
reloaded, this code is no longer necessary for the plugin to function.

Trace related to crash:

```
Application Specific Information:
*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Invalid parameter not satisfying: item != nil'

Last Exception Backtrace:
0   CoreFoundation                       0x00007fff8546a634 __exceptionPreprocess + 148
1   libobjc.A.dylib                      0x00007fff921486de objc_exception_throw + 43
2   CoreFoundation                       0x00007fff8546a42a +[NSException raise:format:arguments:] + 106
3   Foundation                           0x00007fff9441b5b9 -[NSAssertionHandler handleFailureInMethod:object:file:lineNumber:description:] + 195
4   AppKit                               0x00007fff857f04f1 -[NSMenu removeItem:] + 135
5   TXURLDumper                          0x000000010ad5831e -[TXURLDumper pluginWillBeUnloadedFromMemory] + 122
6   Textual 5                            0x00000001000dc813 -[THOPluginItem sendDealloc] (THOPluginItem.m:273)
7   Textual 5                            0x00000001000d3078 __33-[THOPluginManager unloadPlugins]_block_invoke (THOPluginManager.m:144)
8   libdispatch.dylib                    0x00007fff8e6bfc13 _dispatch_client_callout + 8
9   libdispatch.dylib                    0x00007fff8e6c0e5e _dispatch_barrier_sync_f_invoke + 57
10  Textual 5                            0x00000001000097d4 TXPerformBlockSynchronouslyOnQueue (TXGlobalModels.m:396)
11  Textual 5                            0x00000001000d2f7e -[THOPluginManager unloadPlugins] (THOPluginManager.m:142)
12  Textual 5                            0x000000010000ac8a -[TXMasterController performApplicationTerminationStepOne] (TXMasterController.m:355)
13  libdispatch.dylib                    0x00007fff8e6c4323 _dispatch_call_block_and_release + 12
14  libdispatch.dylib                    0x00007fff8e6bfc13 _dispatch_client_callout + 8
15  libdispatch.dylib                    0x00007fff8e6cbcbf _dispatch_main_queue_callback_4CF + 861
16  CoreFoundation                       0x00007fff853bdc59 __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 9
17  CoreFoundation                       0x00007fff8537a2ef __CFRunLoopRun + 2159
18  CoreFoundation                       0x00007fff85379838 CFRunLoopRunSpecific + 296
19  HIToolbox                            0x00007fff869b243f RunCurrentEventLoopInMode + 235
20  HIToolbox                            0x00007fff869b21ba ReceiveNextEventCommon + 431
21  HIToolbox                            0x00007fff869b1ffb _BlockUntilNextEventMatchingListInModeWithFilter + 71
22  AppKit                               0x00007fff857e7821 _DPSNextEvent + 964
23  AppKit                               0x00007fff857e6fd0 -[NSApplication nextEventMatchingMask:untilDate:inMode:dequeue:] + 194
24  AppKit                               0x00007fff85af0f52 -[NSApplication _shouldTerminate] + 993
25  AppKit                               0x00007fff85a06f99 -[NSApplication terminate:] + 729
26  libsystem_trace.dylib                0x00007fff8ec0fcd7 _os_activity_initiate + 75
27  AppKit                               0x00007fff859b65e7 -[NSApplication sendAction:to:from:] + 410
28  AppKit                               0x00007fff859d072a -[NSMenuItem _corePerformAction] + 382
29  AppKit                               0x00007fff859d0447 -[NSCarbonMenuImpl performActionWithHighlightingForItemAtIndex:] + 114
30  libsystem_trace.dylib                0x00007fff8ec0fcd7 _os_activity_initiate + 75
31  AppKit                               0x00007fff859cf47f -[NSMenu performKeyEquivalent:] + 359
32  AppKit                               0x00007fff859cba15 -[NSApplication _handleKeyEquivalent:] + 920
33  AppKit                               0x00007fff8594e8eb -[NSApplication sendEvent:] + 4035
34  AppKit                               0x00007fff857dafe8 -[NSApplication run] + 711
35  AppKit                               0x00007fff857c6424 NSApplicationMain + 1832
36  Textual 5                            0x00000001000e6231 main (main.m:7)
37  libdyld.dylib                        0x00007fff8e0dc5c9 start + 1
38  ???                                  0x0000000000000001 0x0 + 0
```