core: Remove Core::CurrentProcess()

This only encourages the use of the global system instance (which will
be phased out long-term). Instead, we use the direct system function
call directly to remove the appealing but discouraged short-hand.

src/core/core.h

@@ -359,8 +359,4 @@ private:
     static System s_instance;
 };
 
-inline Kernel::Process* CurrentProcess() {
-    return System::GetInstance().CurrentProcess();
-}
-
 } // namespace Core