selinuxrelabel: Remove attempts to relabel /system

These are a no-op due to the additional block-device protection from
https://android.googlesource.com/platform/system/core/+/e18c0d508a6d8b4376c6f0b8c22600e5aca37f69
so get rid of them entirely. Any installing recovery (or equivalent
installation process) should handle the labeling of system components
on its own.

Change-Id: I14c50f08efa6f05b03fad4fbc404219cc504e1ff
TBD: Do we still want to label unlabeled files?
Revert "selinuxrelabel: Remove attempts to relabel /system"

This reverts commit 194847efe28659191541dec87b0e1df8e8894c21.
Revert "Revert "selinuxrelabel: Remove attempts to relabel /system""

This reverts commit 8b4ecff4108cf3e21745afa8b82ed1d34e239669.