Remove flag compatchange_for_zerojankproxy

This was originally added in [1], however with some recent changes in
[2] the boolean derived from this is only passed, but never read.

  [1]: I6b8787a2ada0b4a60e6b448b674d05e172e594a7
  [2]: I2b04517af59b949d30929db577c288b7232cfe65

Flag: EXEMPT removal of android.view.inputmethod.compatchange_for_zerojankproxy
Test: atest CtsInputMethodTestCases
Bug: 409604900
Change-Id: I286af7220794a6af788ec3b8273ea602b7ca8b43