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: I7fffe2fd12f586ed9b3ae54e35d17abdebbe7bce