Remove IInputMethodManager#finishInput(), which is NOP

Currently InputMethodManagerService#finishInput() does nothing. Until
we fully understand what is the right approach on how and when
InputMethodService#finishInput() (Bug 9216494), let's remove this
unnecessary IPC from the IME client to InputMethodManagerService.

Bug: 9216494
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Test: atest FrameworksCoreTests:android.view.inputmethod.InputMethodManagerTest
Change-Id: I614050d20f4a7d9611dc0502e55e6ca3458a836e