Remove SELinux enforcing check from init scripts

The default SELinux policy for RHEL and Fedora has been updated
to include ZFS in the list of filesystems which support xattrs.
Therefore, there's no longer a need to detect this in the init
scripts.

References:

https://bugzilla.redhat.com/show_bug.cgi?id=811532
https://bugzilla.redhat.com/show_bug.cgi?id=816543

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2166