auth: remove unused forward declaration

`timeout_config` is not used by auth/common.hh. presumably, this
class is not a public interface exposed by auth, as it is not
inherently related auth. timeout_config is a shared setting across
related services, specifically, redis_server, thrift and cql_server.
so, in this change, let's drop this forward declaration.

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

auth/common.hh

@@ -30,8 +30,6 @@ namespace replica {
 class database;
 }
 
-class timeout_config;
-
 namespace service {
 class migration_manager;
 }