s390/disassembler: remove insn_to_mnemonic()

insn_to_mnemonic() was introduced ages ago for KVM debugging, but is
unused in the meantime. Therefore remove it.

Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>