Remove reachable call to pg_unreachable().

The loop just before this uses break, not return, so this line
is reachable.  Commit cafe105655
introduced this issue.

Jelte Fennema-Nio, reviewed by Tristan Partin

Discussion: http://postgr.es/m/CAGECzQTO72jKed5461W8cytV2Msh_e+WUZjOyX_RUQCbjk4LRA@mail.gmail.com

src/bin/psql/command.c

@@ -3815,8 +3815,6 @@ wait_until_connected(PGconn *conn)
 				pg_unreachable();
 		}
 	}
-
-	pg_unreachable();
 }
 
 void