SystemMonitor: Don't invalidate ProcessStateModel on refresh

The model has a fixed number of rows based on the column enum
and shouldn't need to be invalidated every update.

Userland/Applications/SystemMonitor/ProcessStateWidget.cpp

@@ -72,7 +72,7 @@ public:
                 break;
             }
         }
-        invalidate();
+        did_update(GUI::Model::UpdateFlag::DontInvalidateIndices);
     }
 
 private: