Touchpad: remove individual gesture settings

These were never plumbed in, never mind launched, and are sometimes
causing confusion in design discussions when people have the feature
flag turned on. Let's remove them.

Bug: 245989146
Test: check System > Touchpad > Touchpad gestures option is removed even
      when the flag's enabled
Flag: EXEMPT removing code controlled by a legacy sysprop-based flag
Change-Id: I22ca19faf6fed430bb3f0733867716a02514ac8d