narrow: remove assertion about working copy being clean
The user can always modify the working copy, including while they're
running `hg tracked --remove-include <path>`. Nothing really bad
happens when they do that, and we already have code for printing a
nice warning, so we can safely remove the assertion we had.
Differential Revision: https://phab.mercurial-scm.org/D10063
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.