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
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL