Remove rugged_backend struct that is no longer needed or used.

Author: arthurschreiber

ext/rugged/rugged.h

@@ -191,10 +191,4 @@ static inline VALUE rugged_create_oid(const git_oid *oid)
 	return rb_str_new(out, 40);
 }
 
-
-typedef struct _rugged_backend {
-  int (* odb_backend)(git_odb_backend **backend_out, struct _rugged_backend *backend, const char* path);
-  int (* refdb_backend)(git_refdb_backend **backend_out, struct _rugged_backend *backend, const char* path);
-} rugged_backend;
-
 #endif