doc: make gendoc.py module import policy aware
Without this, running gendoc.py during an install without C modules
available (via `make local`) will result in an import failure because
the default module load policy insists on C modules.
We also remove the sys.path adjustment because it is no longer needed
since our magic importer handles things.
https://bz.mercurial-scm.org/1089
$ hg init
$ mkdir a
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b (glob)
$ hg ci -m m a
$ mkdir a b
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b (glob)
$ cd b
Relative delete:
$ hg ci -m m ../a
$ cd ..