gas: adjust impossible/bogus M68K/MRI special case when scrubbing

State 1 is uniformly handled further up. And it is highly questionable
that in state 10 (i.e. after having seen not only a possible label, but
also an opcode), which is about to go away anyway, a line comment char
could still be meant to take effect. With the state checking dropped,
the immediately preceding logic can then also be simplified.