s390/cpum_cf: remove in-kernel counting facility interface

Commit 17bebcc6

 ("s390/cpum_cf: Add minimal in-kernel interface for
counter measurements") introduced a small in-kernel interface for CPU
Measurement counter facility.
There are no users of this interface, therefore remove it.

The following functions are removed:
 kernel_cpumcf_alert(),
 kernel_cpumcf_begin(),
 kernel_cpumcf_end(),
 kernel_cpumcf_avail()
there is no need for them anymore.
With the removal of function kernel_cpumcf_alert(), also remove
member alert in struct cpu_cf_events. Its purpose was to counter
measurement alert interrupts for the in-kernel interface.

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>