parsers: remove long-dead parse_manifest method We haven't used this in years, I think it's fine to ditch it now. We had previously kept it around to ease bisecting with built extensions, but these days we've got a better versioning scheme anyway. Noticed this method kicking around while looking in parsers.so for likely fuzzing targets. Differential Revision: https://phab.mercurial-scm.org/D5459