src: remove unused data member write_queue_size_

Remove TLSWrap::write_queue_size_, it's not used anywhere.

PR-URL: #7374
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

src/tls_wrap.h

@@ -150,7 +150,6 @@ class TLSWrap : public AsyncWrap,
   BIO* enc_out_;
   NodeBIO* clear_in_;
   size_t write_size_;
-  size_t write_queue_size_;
   typedef ListHead<WriteItem, &WriteItem::member_> WriteItemList;
   WriteItemList write_item_queue_;
   WriteItemList pending_write_items_;