Userland: Prefer _string over _short_string

As `_string` can't fail anymore (since 3434412), there are no real
benefits to use the short variant in most cases.

Userland/Applications/SystemMonitor/ThreadStackWidget.cpp

@@ -34,11 +34,11 @@ public:
     {
         switch (column) {
         case Column::Address:
-            return "Address"_short_string;
+            return "Address"_string;
         case Column::Object:
-            return "Object"_short_string;
+            return "Object"_string;
         case Column::Symbol:
-            return "Symbol"_short_string;
+            return "Symbol"_string;
         default:
             VERIFY_NOT_REACHED();
         }