service/ir: Remove erroneous forward declaration

This can be removed as it's not used. Even if it were however, it would
be an incorrect forward declaration, as ServiceManager exists within the
Service::SM namespace, not the top-level SM namespace.

src/core/hle/service/ir/ir.h

@@ -8,10 +8,6 @@ namespace Core {
 class System;
 }
 
-namespace SM {
-class ServiceManager;
-}
-
 namespace Service::IR {
 
 void InstallInterfaces(Core::System& system);