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
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