InputDispatcher: clean up input event tracing flag

This launched ages ago.

Test: take a trace using the adb shell perfetto command, open it in
      Winscope and check input events are present
Fixes: 409605937
Flag: EXEMPT removing com.android.input.flags.enable_input_event_tracing
Change-Id: Ia0427592a4ea196acb16aa2b34cbf74988b837dc