Touchpad: remove tap dragging flag

This feature has now launched, so we can remove the flag.

Bug: 349562564
Test: toggle tap dragging setting, check tap dragging works when it's
      enabled but not when it's disabled
Flag: EXEMPT removing com.android.hardware.input.touchpad_tap_dragging
Change-Id: I6454b57b97f93e5cccfafc784f3fdc63a16f80ed