Remove unused /selinux_version

This file is no longer needed because it was needed for supporting
reloadable/dynamic SELinux policy which is no longer supported.

Test: Clean build, flash, device boots without additional denials.
      Reboot to recovery works, no additional denials.
Bug: 33642277
Change-Id: Ie65e74b8c8437694bc1ea019ab844f60f9e89ade