gas: there's no scrubber state 12

Apparently (beyond what's [easily] visible in git history) when this was
added there was confusion about scrubber states vs lex[] contents. For
the purposes here LEX_IS_DOUBLEDASH_1ST (which happens to also resolve
to 12) alone is sufficient. "state" is never set to 12, and it being 12
also isn't handled anywhere.