LibJS: Remove unused ThrowCompletionOr<T>::release... helper

Author: https://github.com/awesomekling
Commit: https://github.com/LadybirdBrowser/ladybird/commit/c0600c43536
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/4232

Libraries/LibJS/Runtime/Completion.h

@@ -301,12 +301,6 @@ public:
         return Completion { Completion::Type::Throw, error.error };
     }
 
-    ValueType release_allocated_value_but_fixme_should_propagate_errors()
-    {
-        VERIFY(!is_error());
-        return release_value();
-    }
-
 private:
     Variant<ValueType, ErrorValue> m_value_or_error;
 };