Remove settings for individual touchpad gestures

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 the feature flag no longer appears in Developer Options >
      Feature flags
Flag: EXEMPT removing code controlled by a legacy sysprop-based flag
Change-Id: I365854a36fd7d210b2c9f93a373fa70e15e91a6a