powerpc/xmon: Do not compute/store the major opcode

In commit 5b102782 ("powerpc/xmon: Enable disassembly files (compilation
changes)") usage of variable `op` has been removed. Completely remove opcode
computation since not used anymore.

Fix fatal warning:

arch/powerpc/xmon/ppc-dis.c: In function ‘lookup_powerpc’:
arch/powerpc/xmon/ppc-dis.c:96:17: error: variable ‘op’ set but not used [-Werror=unused-but-set-variable]
   unsigned long op;
                 ^~

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>