util: remove unused ctxmanager
This was meant as a substitute for Python's "with" with multiple
context managers before we moved to Python 2.7. We're now on 2.7, so
we should have no reason to keep ctxmanager. "hg grep --all
ctxmanager" says that it was never used anyway.
Differential Revision: https://phab.mercurial-scm.org/D73
pager.pager=p0 # $TESTTMP/sysrc:4
ui.editor=e1 # $TESTTMP/userrc:2
pager.pager=p2 # $PAGER
ui.editor=e1 # $TESTTMP/userrc:2