Remove InputMethodManager.ImeThreadFactory, which is unused

InputMethodManager.ImeThreadFactory was originally introduced to fix
Bug 147331480 but all its usages were already removed by a subsequent
CL [2], which somehow forgot to remove this class.

Anyways, this class is definitely unused right now hence it should be
safe for us to remove it.

 [1]: I10391b834c33458c3e0ac846ab738e805d235c26
      ba400449
 [2]: Id73f0d5efb68bb4c5c8ef3dfb974c7a4f9457581
      08e91345

Bug: 171827834
Test: presubmit
Change-Id: I4d1a24be1272670b89b1435f6fd6ff196a22385f