Remove convenience methods for CONNECT verb. (#177)

CONNECT, specified at https://tools.ietf.org/html/rfc7231#section-4.3.6,
says:

> if successful, thereafter restrict its behavior to blind forwarding
> of packets, in both directions, until the tunnel is closed.

ureq doesn't actually support the semantics of CONNECT, since it doesn't
offer a bidirectional channel on a Response. So I'm fairly confident no
one is using these methods.

src/lib.rs

@@ -199,11 +199,6 @@ pub fn options(path: &str) -> Request {
     request("OPTIONS", path)
 }
 
-/// Make an CONNECT request.
-pub fn connect(path: &str) -> Request {
-    request("CONNECT", path)
-}
-
 /// Make an PATCH request.
 pub fn patch(path: &str) -> Request {
     request("PATCH", path)