Delete virtualenv faq.

[anntzer]

The situation has basically improved to the point where it's just "it
works, though installing some GUI toolkits may be a bit tricky."
Mention these tricky points in INSTALL.rst instead.

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[timhoffm]

Looks ok, but would like a second opinion even though this is just doc change.