Minor simplification of (Python) find_thread_object

Minor simplification of (Python) find_thread_object

Since the reference to the Inferior Python object is managed by
gdbpy_ref (RAII), we can return directly from the loop.  It's just a
leftover from the cleanups era.

gdb/ChangeLog:

	* python/py-inferior.c (find_thread_object): Return directly
	from the loop.  Remove "found" variable.