narrowchangegroup: remove backwards compatibility with old hg
This was missed in the initial import of narrowhg, but was detected by
the Python 3 porting effort once I got enough other things in narrow
fixed.
Differential Revision: https://phab.mercurial-scm.org/D2370
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.