narrow: remove unused "cacheprop" stuff
This is unused since D3046. I didn't even notice it then, but tests
still pass so I hope it's still handled safely.
Differential Revision: https://phab.mercurial-scm.org/D3142
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.