lowmem: Remove the possibility to stack the module

The mod_reg_security() function that is used to stack modules has been removed:
"LSMs wishing to stack capability as a secondary module should do so
explicitly". Thus, let's remove the possibility to stack in the init function.
The functionality could probably be reintroduced later if somebody has time to
implement it.

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