s390/smp: get rid of generic_smp_call_function_interrupt

Since 9a46ad6d "smp: make smp_call_function_many() use logic similar
to smp_call_function_single()" generic_smp_call_function_interrupt()
is only an alias to generic_smp_call_function_single_interrupt().

So remove the superfluous variant.

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