virtualenv.el integration: Remove.

This removes the advices in elpy.el which would offer to restart the
elpy-rpc backend when the virtualenv changes. This integration was
pretty frickle to begin with, and makes even less sense now with
per-project RPC buffers. So we just get rid of it completely.
New defcustom: elpy-show-installation-instructions

Prevents the *Elpy Installation* buffer from popping up.