LibJS: Remove direct argument loading since it was buggy

The parser doesn't always track lexical scopes correctly, so let's not
rely on that for direct argument loading.

This reverts the LoadArguments bytecode instruction as well. We can
bring these things back when the parser can reliably tell us that
a given Identifier is indeed a function argument.

Userland/Libraries/LibJS/Bytecode/Op.h

@@ -651,22 +651,6 @@ private:
     HashMap<u32, Variable> m_variables;
 };
 
-class LoadArgument final : public Instruction {
-public:
-    explicit LoadArgument(size_t index)
-        : Instruction(Type::LoadArgument)
-        , m_index(index)
-    {
-    }
-
-    void execute_impl(Bytecode::Interpreter&) const;
-    String to_string_impl(Bytecode::Executable const&) const;
-    void replace_references_impl(BasicBlock const&, BasicBlock const&) { }
-
-private:
-    size_t m_index { 0 };
-};
-
 class GetIterator final : public Instruction {
 public:
     GetIterator()