Remove 'trialcoverage' plugin and support code

Closes ticket:2281 (trac).

This removes src/allmydata/test/trial_coverage.py, which was a
in-process way to run trial tests under the "coverage" code-coverage
tool. These days, the preferred way to do this is with "coverage run",
although the actual invocation is a bit messy because of the way
bin/trial uses subprocess.call() to invoke the real entrypoint script
with the right PYTHONPATH (see #1698 for details). Hopefully this will
be improved to use a simpler "coverage run .." command in the future.

This patch also removes twisted/plugins/allmydata_trial.py, which
enabled the "--reporter=bwverbose-coverage" option. Finally it modifies
setup.py to stop looking for that option and adding "trialcoverage" to
the dependencies list, which gets us closer to removing "setup_requires"
entirely.