LibJS: Mark classes and virtual functions final where possible

These cases were found with GCC's `-Wsuggest-final-{types,methods}`
warnings, which catch calls that could have been devirtualized had we
declared the functions `final` in the source.

To reproduce, Link Time Optimization needs to be enabled. The easiest
way to achieve this is to set the `CMAKE_INTERPROCEDURAL_OPTIMIZATION`
cache variable to `ON`. The `.incbin` directive in LibCompress' Brotli
decompressor might needs to be changed to an absolute path for this to
work.

This commit also removes a pair of unused virtual functions.

Userland/Libraries/LibJS/AST.h

@@ -156,7 +156,7 @@ public:
 };
 
 // 14.13 Labelled Statements, https://tc39.es/ecma262/#sec-labelled-statements
-class LabelledStatement : public Statement {
+class LabelledStatement final : public Statement {
 public:
     LabelledStatement(SourceRange source_range, DeprecatedFlyString label, NonnullRefPtr<Statement const> labelled_item)
         : Statement(move(source_range))