Remove IMM#getClient(), which is unused

InputMethodManager#getClient() is no longer marked as
@UnsupportedAppUsage [2] hence we should be able to remove it as far
as we can see.

Let's see how it goes.

  [1]: I47f3a5438bad6c8fa386b68ae7ac918a5c3e5d71
       835bc550

Bug: 192412909
Bug: 194333155
Test: presubmit
Change-Id: I1cfba4f4bd73672db7d264499f72546fc96db62f