setup: comment out the setup_requires on setuptools_darcs in order to work-around...

setup: comment out the setup_requires on setuptools_darcs in order to work-around distribute bug 55
Note that we still inject the setuptools_darcs egg at the beginning of setup.py, which makes the setuptools_darcs plugin work when building dists from tahoe-lafs source so those dists have all of the needed files.
http://bitbucket.org/tarek/distribute/issue/55/revision-control-plugin-automatically-installed-as-a-build-dependency-is-not-present-when-another-build-dependency-is-being