Fix UITableView observation info getting leaked

`ARGenericTableViewController` is added as an observer of its table view's `editing` keypath within `-viewDidLoad`, but is never removed. When the `UITableView` is deallocated we have a hanging observer – the observer may get attached to some other object.

This is the source of the following console warning that pops up when a `ARGenericTableViewController` goes away:

```
An instance 0x130a2000 of class UITableView was deallocated while key value observers were still registered with it. Observation info was leaked, and may even become mistakenly attached to some other object. Set a breakpoint on NSKVODeallocateBreak to stop here in the debugger. Here's the current observation info:
<NSKeyValueObservationInfo 0x11a61280> (
<NSKeyValueObservance 0x11a61220: Observer: 0x11a3d0c0, Key path: editing, Options: <New: NO, Old: NO, Prior: NO> Context: 0x0, Property: 0x11a612a0>
)
```