Remove 'git seems stuck' and 'diagnostics for rvm' pages from installfest

* The "what to do if git seems stuck" page is outdated, a little
strange, and windows-specific.
* The 'rvm diagnostics' page doesn't provide students with much
information, it just gives TAs a place to start.

In both situations, it's probably better to just leave it up to the
judgement of the TA.