Remove IMM#getInputContext(), which is unused

InputMethodManager#getInputContext() is no longer marked as
@UnsupportedAppUsage [1] hence we should be able to remove it as far
as we can see.

Let's see how it goes.

 [1]: Id83ce2bd5896e953c824f7eaf28b235859123ab0
      51c93504

Bug: 192412909
Bug: 194323038
Test: presubmit
Change-Id: I6069fbb3b40e51bb24bfece48a4bb56705ded6ba