Libraries: Use default constructors/destructors in LibWeb

https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#cother-other-default-operation-rules

"The compiler is more likely to get the default semantics right and
you cannot implement these functions better than the compiler."

Userland/Libraries/LibWeb/CSS/MediaQuery.h

@@ -205,7 +205,7 @@ struct MediaCondition {
     String to_string() const;
 
 private:
-    MediaCondition() { }
+    MediaCondition() = default;
     Type type;
     Optional<MediaFeature> feature;
     NonnullOwnPtrVector<MediaCondition> conditions;