s390/disassembler: remove confusing code

s390/disassembler: remove confusing code

When searching the opcode offset table within find_insn() the check
"entry->opcode == 0" was intended to clarify that 1-byte opcodes, the
first one being 0, are special.

However there is no mnemonic for an illegal opcode starting with 0.
Therefore there is also no opcode offset table entry that matches,
which again means that the check never is true. Therefore just remove
the confusing check, and add a comment which hopefully explains how
this works.

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