SELinux: Remove security_get_policycaps()

The security_get_policycaps() functions has a couple of bugs in it and it
isn't currently used by any in-tree code, so get rid of it and all of it's
bugginess.

Signed-off-by: Paul Moore <paul.moore@hp.com>
Signed-off-by: James Morris <jmorris@localhost.localdomain>