Revert "Add a null home activity to Shell.apk"

This reverts commit 6b1c4a9e.

Bug: 149226068
Bug: 149307007
Bug: 149780604
Test: $ lunch aosp_crosshatch-userdebug; m
      # flash and boot on a crosshatch
      # Settings -> Apps & notifications -> Default apps -> Home apps
      # Only Quickstep home is there; no Shell home any more
      $ lunch csi_arm64-userdebug; m
      # flash the resulted system.img on a crosshatch and boot
      $ atest DeviceHealthTests # all passed
Change-Id: I927b7dae779da25589f2198e0480c0f2a788a7e4
Merged-In: I927b7dae779da25589f2198e0480c0f2a788a7e4
(cherry picked from commit 539298c1)