KVM: s390: remove unused gisa_clear_ipm_gisc() function

clang warns about an unused function:
arch/s390/kvm/interrupt.c:317:20:
  error: unused function 'gisa_clear_ipm_gisc' [-Werror,-Wunused-function]
static inline void gisa_clear_ipm_gisc(struct kvm_s390_gisa *gisa, u32 gisc)

Remove gisa_clear_ipm_gisc(), since it is unused and get rid of this
warning.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Link: https://lore.kernel.org/r/20221118151133.2974602-1-hca@linux.ibm.com
Signed-off-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>