Remove IInputMethodSession#finishInput(), which is called nowhere

Currently IInputMethodSession#finishInput() is called nowhere.

Let's remove this IPC method exposed from InputMethodService to IME
client for now until we fully understand what is the right approach on
how and when InputMethodService#finishInput() should be called.

Note that we cannot simply remove InputMethodSession#finishInput()
because it is already published as a public API.  In Bug 9216494
hopefully we can also discuss whether InputMethodSession#finishInput()
should be deprecated or can be reused for some actual use cases.

Bug: 9216494
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Change-Id: I9e378111c9df40cb6821583d1e6ae8f9fe38f2b1