rx: Remove obsolete comment

This comment refers to the fact that we used to be just checking for
SELinux to see if we should pass that extra argument. Ever since
commit cb1b41b159b98881f66319d7f65d941ba9fab911, we do have a better
test for this.

Change-Id: Idf2ff879f05774f49a11d04f87579afccf385b57
Reviewed-on: http://gerrit.openafs.org/10593
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>