Remove redundant swipe down notification and one handed sub settings

1. Remove "Swipe down for notification" item code.
2. Remove "Exit when switching apps & "Timeout" items code.

Bug: 175851783

Test: build & verify Settings > System > Gestures
Test: make RunSettingsRoboTests ROBOTEST_FILTER=
      "com.android.settings.gestures
      .OneHandedEnablePreferenceControllerTest"

Change-Id: I77be5ff5542f6310fa0b9649ddfc1d3c16fa4261