urlprotocol: remove unused url_interrupt_cb declaration

It is a remnant of the old interrupt callback API.

libavformat/url.h

@@ -34,8 +34,6 @@
 #define URL_PROTOCOL_FLAG_NESTED_SCHEME 1 /*< The protocol name can be the first part of a nested protocol scheme */
 #define URL_PROTOCOL_FLAG_NETWORK       2 /*< The protocol uses network */
 
-extern int (*url_interrupt_cb)(void);
-
 extern const AVClass ffurl_context_class;
 
 typedef struct URLContext {