LibJS: Remove unused InstructionStreamIterator::source_code getter

Author: https://github.com/ADKaster
Commit: 59b1fb23a9
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/4362
Reviewed-by: https://github.com/AtkinsSJ
Reviewed-by: https://github.com/Hendiadyoin1

Libraries/LibJS/Bytecode/Instruction.h

@@ -205,7 +205,6 @@ public:
     }
 
     UnrealizedSourceRange source_range() const;
-    RefPtr<SourceCode> source_code() const;
 
     Executable const* executable() const { return m_executable; }