[train] remove DEFAULT_NCCL_SOCKET_IFNAME

[matthewdeng]

Why are these changes needed?

This was originally added in #28633, #31824 in an attempt to set sensible default configurations. After further investigating, it appears that users actually expect (and prefer) the default unset values.

This PR also removes additional configs set in release tests, as the test environments have already been configured properly.

Related issue number

Closes #42652
Closes #32519

Checks

Release Tests (See AWS tests): https://buildkite.com/ray-project/release/builds/7192

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[woshiyyya]

Looks good! Thanks for the cleanup. Let's merge it.