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
#require test-repo jshint hg10
$ . "$TESTDIR/helpers-testrepo.sh"
run jshint on all tracked files ending in .js except vendored dependencies
$ cd "`dirname "$TESTDIR"`"
$ testrepohg locate 'set:**.js' \
> 2>/dev/null \
> | xargs jshint