Revert "add IRCBot.http_client(), to allow modules to override default http client"

This reverts commit 81347fbba0.

src/IRCBot.py

@@ -59,9 +59,6 @@ class Bot(object):
 
         self._events.on("timer.reconnect").hook(self._timed_reconnect)
 
-    def http_client(self):
-        return self._exports.get_one("http-client", utils.http.Client)
-
     def _trigger_both(self):
         self.trigger_read()
         self.trigger_write()