Remove "Built on Pure" section from the Extend page
Remove "Built on Pure" section from the Extend page

These modules are not really built on Pure and it seems better to remove
the reference from them now util the Bower enhancements land and we use
that to extend Pure.

Closes #125

jamesalley/pure-site,jamesalley/pure-site,weigang992003/pure-css-chinese,yahoo/pure-site,weigang992003/pure-css-chinese,yahoo/pure-site