API Remove Trackback support

It was disabled since migrating the module to SS3,
and the whole concept has become less relevant since spammers discovered trackbacks
(ref http://en.wikipedia.org/wiki/Trackback#Spam).

Since we don't have the resources to make a spam-proof trackback system,
its prudent to remove it instead.
Merge pull request #81 from PutmanMedia/pulls/remove-trackback

API Remove Trackback support