Remove "Prior art" section from README

These are very old projects from another time. Today, it doesn't make
sense to try to list similar projects, as they are easily discoverable
through other means. In the recent years, hub hasn't taken direct
inspiration from projects other than git.

Closes #1345

README.md

@@ -104,11 +104,3 @@ Meta
 * Home: <https://github.com/github/hub>
 * Bugs: <https://github.com/github/hub/issues>
 * Authors: <https://github.com/github/hub/contributors>
-
-### Prior art
-
-These projects also aim to either improve git or make interacting with
-GitHub simpler:
-
-* [eg](http://www.gnome.org/~newren/eg/)
-* [github-gem](https://github.com/defunkt/github-gem)