s390/facilities: get rid of __ASSEMBLY__ in facility header file

There is no need for the __ASSEMBLY__ ifdefery anymore since the
architecture level set code that deals with facility bits was
converted to C in the meantime.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>