LibJS+LibWeb: Remove more uses of DeprecatedFlyString

Author: https://github.com/awesomekling
Commit: f1914893e9
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/4067
Reviewed-by: https://github.com/trflynn89

Libraries/LibJS/Bytecode/Op.h

@@ -1336,8 +1336,6 @@ public:
     Operand base() const { return m_base; }
     Operand property() const { return m_property; }
 
-    Optional<DeprecatedFlyString const&> base_identifier(Bytecode::Interpreter const&) const;
-
 private:
     Operand m_dst;
     Operand m_base;