remove unused InternalIteratorBase::is_mutable_ (#11104)

Summary:
`InternalIteratorBase::is_mutable_` is not used any more, remove it.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11104

Reviewed By: ajkr

Differential Revision: D42582747

Pulled By: cbi42

fbshipit-source-id: d30bf75151fc8414df0ae112a6ec4943b5b7330b

table/internal_iterator.h

@@ -203,8 +203,6 @@ class InternalIteratorBase : public Cleanable {
       Prev();
     }
   }
-
-  bool is_mutable_;
 };
 
 using InternalIterator = InternalIteratorBase<Slice>;