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/hle/kernel/handle_table.cpp

@@ -103,7 +103,7 @@ SharedPtr<Object> HandleTable::GetGeneric(Handle handle) const {
     if (handle == CurrentThread) {
         return GetCurrentThread();
     } else if (handle == CurrentProcess) {
-        return Core::CurrentProcess();
+        return Core::System::GetInstance().CurrentProcess();
     }
 
     if (!IsValid(handle)) {