Problem: out_batch_size has been replaced by ZMQ_TCP_SEND_BUFFER option in cdeec4c1

Solution: remove `out_batch_size` from src/config.hpp

src/config.hpp

@@ -59,12 +59,6 @@ namespace zmq
         //  unnecessary network stack traversals.
         in_batch_size = 8192,
 
-        //  Maximal batching size for engines with sending functionality.
-        //  So, if there are 10 messages that fit into the batch size, all of
-        //  them may be written by a single 'send' system call, thus avoiding
-        //  unnecessary network stack traversals.
-        out_batch_size = 8192,
-
         //  Maximal delta between high and low watermark.
         max_wm_delta = 1024,