overlay: Remove back button panic overlay

 * Google decided to kill the feature.
   See the related fw/base commit for reference:
   https://github.com/LineageOS/android_frameworks_base/commit/bb97879c959c15c61f86b8465d583b80a21626b1

Change-Id: Iaa85020c4b2ab69d4e7cf7415e04e08990a9ea4d