Remove GlanceableHubBackGesture feature.

This changelist removes handling the back gesture on glanceable hub.
The communication of glanceable hub's presence to launcher through
the OverviewProxyService is preserved for use by other features.

Test: manual - verified navigation to and from Glanceable Hub worked as
      expected without the flag.
Bug: 361822449
Flag: EXEMPT removing com.android.systemui.glanceable_hub_back_gesture
Change-Id: I956432a012a15a7eb7b5fb58aa741ff1fd4b489e