[C10D] Remove watchKey functionality from the Store. (#105014)
The feature was never fully finished and never got any adoption but
TCPStore pays the cost of twice the number of tcp connections anyway.
While the cost of all those idle connections is minimal is doesn't come for free:
- It increases the likelyhood of a connection refused failure during the initialization stampede.
- TCPStore uses poll for checking for socket availability which scales linearly on the number of sockets regardless of their status.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/105014
Approved by: https://github.com/fduwjj
