CLEANUP: mux: remove the unused "release()" function

In commit 53a4766 ("MEDIUM: connection: start to introduce a mux layer
between xprt and data") we introduced a release() function which ends
up never being used. Let's get rid of it now.

include/types/connection.h

@@ -296,7 +296,6 @@ struct mux_ops {
 	void (*shutr)(struct conn_stream *cs, enum cs_shr_mode);     /* shutr function */
 	void (*shutw)(struct conn_stream *cs, enum cs_shw_mode);     /* shutw function */
 
-	void (*release)(struct connection *conn);     /* release all resources allocated by the mux */
 	struct conn_stream *(*attach)(struct connection *); /* Create and attach a conn_stream to an outgoing connection */
 	void (*detach)(struct conn_stream *); /* Detach a conn_stream from an outgoing connection, when the request is done */
 	char name[8];                                 /* mux layer name, zero-terminated */

src/mux_h2.c

@@ -3170,7 +3170,6 @@ const struct mux_ops h2_ops = {
 	.detach = h2_detach,
 	.shutr = h2_shutr,
 	.shutw = h2_shutw,
-	.release = h2_release,
 	.name = "H2",
 };