input: clean up touchpad visualizer flag

This launched in Android 16, so we can safely clean it up now.

Bug: 388224751
Test: connect a touchpad, turn the "Show touchpad input" dev. option
      on, check the visualization works, and turn it off again.
Flag: EXEMPT removing com.android.hardware.input.touchpad_visualizer
Change-Id: Ib2112b7aa4c0f95ba56c30cc7e033d01ac8098f3