s390/disassembler: Remove duplicate instruction format RSY_RDRU

Instruction format RSY_RDRU is a duplicate of RSY_RURD2. Use the latter,
as it follows the s390-specific conventions for instruction format
naming used in binutils.

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Jens Remus <jremus@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>