Delete unused classes from com.android.internal.inputmethod

This is a follow up CL to my previous CL [1], which forgot to delete
the following files.

 * com.android.internal.inputmethod.IIntResultCallback
 * com.android.internal.inputmethod.ThrowableHolder

Since those classes are no longer used after our switching to
AndroidFuture<T>, there should be no observable behavior changes.

 [1]: I74657826a99b11ca1f86932f8f41cca6e449cc8a
      b2d09de1

Bug: 192412909
Bug: 195699814
Test: presubmit
Change-Id: Ie5076f56c60e3c51fbd374cf74ea4c53f48e5da9