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/FontCache.h

@@ -38,6 +38,6 @@ public:
     void set(const FontSelector&, NonnullRefPtr<Gfx::Font>);
 
 private:
-    FontCache() { }
+    FontCache() = default;
     mutable HashMap<FontSelector, NonnullRefPtr<Gfx::Font>> m_fonts;
 };