HTTP agent: remove excess calls to removeSocket

socket.destroy() triggers a 'close' event from the socket which triggers the onClose handler of HTTPAgent which calls self.removeSocket(). So by calling self.removeSocket() prior to socket.destroy() we end up with two calls to self.removeSocket().

If there are pending requests, removeSocket ends up creating a new socket. So if there are pending requests, each time a request completes, we tear down one socket and create two more. So the total number of sockets grows exponentially and without regard for any maxSockets settings. This was noticed in #4050 (closed). Let's get rid of the extra calls to removeSocket

so we only call it once per completed request.