adb: add lock to remove_socket.

The comment that was previously here says that local_socket_list_lock
must be taken, but this function is exposed to external callers that
can't possibly take the lock.

Bug: http://b/65419665
Bug: 64709603 (presubmit balking at the line above)
Test: python test_device.py
Change-Id: I12d464933936b2a210a827ccf19ea201020d8d78
(cherry picked from commit 62c92f0c)