Remove unused methods and classes from Completable class

This is hopefully the last follow up CL to my previous CL [1], which
aimed to logically revert IInputMethodManager from emulated sync IPCs
to truly sync IPCs.

It turns out that my previous clean-up CL [2] forgot to take care of
the following class

  com.android.internal.inputmethod.Completable,

where several unused methods and classes still existed.

Since those methods and classes are not used right now, there should
be no user/developer visible behavior change in this CL.

 [1]: If16ac0de536d9089eb04f6e07b1ee47378124658
      662b48b7
 [2]: I8666ac1399058b980e51e2459122b2f5f36c77b5
      b1fb07a8

Bug: 192412909
Test: presubmit
Change-Id: Id5d5a4abeff8291b40baf95d425f4035d3cddffe