Remove unused InputMethod#setCurrentShowInputToken()

This is a clean up CL to our previous CL [1], which introduced an
unused @hide method

  InputMethod#setCurrentShowInputToken().

Since that method has never been called, there should be no behavior
change in this CL.

 [1]: Ia49e23dd077d264a58d28a7b8acffde54b7db187
      bb0e2f75

Bug: 111084606
Bug: 192412909
Test: presubmit
Change-Id: Idd607ef62c91a92006da4bf26a86338ea7224ce8