x86: drop bogus accumulator check

Accum is an "instance", not a "class". With present enumerator values of
Reg and Accum, the 2nd check simply did the same as the first. In fact
checking for the accumulator (%rax) isn't necessary here at all, because
there's no case where an individual template would permit alternatively
a memory operand or the (qword) accumulator; only "any GPR" is ever
being paired with "memory".