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?
(cherry picked from commit cae5f245304fe11e86323aa7f6f4c25e898bf36b)