InputSettings: remove touchpad 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
Test: m checkinput
Flag: EXEMPT removing com.android.hardware.input.touchpad_tap_dragging
Change-Id: I0051bb0e9e28ad698279c344f263b97fa159e679