Remove unused back overlay packages

- They were used prior to us migrating to settings-based back
  sensitivity in b/146004827 and was kept for compatibility in case
  users who previously adjusted the sensitivity via the overlay
  packages would retain it upon updating.  However, the changes for
  the above bug went in 2019, and the latest version of Android are
  not supported on these older devices, so we can likely remove them
  entirely now.

Fixes: 332992598
Test: Builds
Change-Id: I1edb24cbd05ef983499abe357bfb756f02f4d915