lowmem: Security modules can no longer be removed.

The unregister_security() and mod_unreg_security() functions were removed
because the upstream developers believe that it should not be possible to remove
security modules, "It is used only by out-of-tree modules, which are often
binary-only, illegal, abusive of the API and dangerous, e.g.  silently
re-vectoring SELinux."

Signed-off-by: Viktor Rosendahl <viktor.rosendahl@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>