Trebuchet: Fix taskbar/3 button navigation SUW height

Taskbar height was broken on tablet in the past few QPRs, now it also
broke on phones since they use taskar.
Explicitly setting gesture navigation doesn't fix anything in SUW, so
just delete all the custom handling and everything works as expected.

Change-Id: I9ba2e320c02981fe75faa7189ca0707208a34814