chore: remove unused constant (#8934)

HOROVOD_GLOO_RENDEZVOUS_PORT is unused.

#2771 was the PR that removed its only use. Once, it was passed directly to horovod.

harness/determined/constants.py

@@ -27,9 +27,6 @@
 
 DTRAIN_SSH_PORT = int(str(os.getenv("DTRAIN_SSH_PORT", "12350")))
 
-# GLOO port used by Horovod for the Gloo controller.
-HOROVOD_GLOO_RENDEZVOUS_PORT = int(str(os.getenv("HOROVOD_GLOO_RENDEZVOUS_PORT", "12355")))
-
 # Port for communicating between training processes. Used for reducing
 # validation metrics.
 INTER_TRAIN_PROCESS_COMM_PORT_1 = int(str(os.getenv("INTER_TRAIN_PROCESS_COMM_PORT_1", "12360")))