Remove unused variable.



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@182892 91177308-0d34-0410-b5e6-96231b3b80d8
<rdar://problem/13956179>

Cleaned up the thread updating code in the OperatingSystemPython class. It doesn't need to clear the "new_thread_list" anymore as it is always empty. 

It also now assigns the "core_thread_list" to "new_thread_list" if no threads are detected through python.



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@182893 91177308-0d34-0410-b5e6-96231b3b80d8