selinux: clean up dead code after removing runtime disable

Commit f22f9aaf ("selinux: remove the runtime disable functionality")
removes the config SECURITY_SELINUX_DISABLE. This results in some dead code
in lsm_hooks.h.

Remove this dead code.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>