1131632 - Remove notes to disable SELinux in EL 5.

I put a lot of effort into making sure we can work with SELinux in
Enforcing mode in EL 5. We can, so this commit removes the documentation
that instructed users to disable it. It also recommends users to
re-enable it upon upgrading in the release notes.

https://bugzilla.redhat.com/show_bug.cgi?id=1131632
Borrowing from your playbook, I don't think we should advise the specific way to enable selinux, because it's more involved than just this command. It should really be enabled in ``/etc/selinux/config``, but there are other steps that should be taken also. Here is a good rhel6 writeup that is probably very accurate for rhel5 also: https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Security-Enhanced_Linux/sect-Security-Enhanced_Linux-Working_with_SELinux-Enabling_and_Disabling_SELinux.html