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