#ifdef out some dead code in psql/mainloop.c.

#ifdef out some dead code in psql/mainloop.c.

This pg_send_history() call is unreachable, since the block it's in
is currently only entered in !cur_cmd_interactive mode.  But rather
than just delete it, make it #ifdef NOT_USED, in hopes that we'll
remember to enable it if we ever change that decision.

Per report from David Binderman.  Since this is basically cosmetic,
I see no great need to back-patch.

Discussion: https://postgr.es/m/HE1PR0802MB233122B61F00A15E035C83BE9C710@HE1PR0802MB2331.eurprd08.prod.outlook.com