input: clean up three-finger tap shortcut flag

Bug: 423845211
Test: with a touchpad connected, check System > Touchpad > Use
      three-finger tap setting still appears and works
Flag: EXEMPT removing
      com.android.hardware.input.touchpad_three_finger_tap_shortcut
Change-Id: Ie3c9a2108ca3c54bba26a24e2c92f080e6ea1eb0