exception: Remove HttpException class

This class served no particular purpose (other than adding a
layer on top of Exception).

lib/Exceptions.php

@@ -11,13 +11,6 @@
  * @link	https://github.com/rss-bridge/rss-bridge
  */
 
-/**
- * Implements a RSS-Bridge specific exception class
- *
- * @todo This class serves no purpose, remove it!
- */
-class HttpException extends \Exception{}
-
 /**
  * Returns an URL that automatically populates a new issue on GitHub based
  * on the information provided