Clean up feature flag for new mouse pointer ballistics

This has launched and so can now be removed.

Bug: 320433834
Test: check pointer movements and adjusting pointer speed in settings
      with a USB mouse
Test: atest frameworks/native/services/inputflinger/tests/CursorInputMapper_test.cpp
Flag: EXEMPT removing
      com.android.input.flags.enable_new_mouse_pointer_ballistics

Change-Id: I6b4241a808993a555232b64aff3a32b01e0b8fef