Remove unused InputMethod#setCurrentHideInputToken()

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

  InputMethod#setCurrentHideInputToken().

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

 [1]: Ia596a392eb73ae46debd097151c8c9a7edd59833
      44e2ae4afc30b6a873e1536e31619db258f6cf00

Bug: 133381284
Bug: 149870112
Bug: 192412909
Test: presubmit
Change-Id: I8906360dc51ad3468b9ce4c97d477ae9bcdb2e91