uwsgi: Remove mono plugin support.

The mono plugin support in uwsgi does not work for several reasons:

1. The mono plugin needs to be linked against the Foundation framework (submitted upstream as https://github.com/unbit/uwsgi/pull/685)
2. Building the mono-plugin has an implicit dependency on pkg-config that isn't mentioned in the Formula (same for the lua plugin, I'll create a new PR for that - unless it's okay if I attach that patch here)
3. The uwsgi plugin dll isn't installed anywhere, so the mono plugin never worked anyway when installed with homebrew. Also, due to a mono bug, the dll currently is *required* to be installed into the global assembly cache, and I'm not sure how that should be handled in a formula (especially the part of uninstalling it when uwsgi is removed).