revert https://github.com/OffchainLabs/prysm/pull/16100 (#16139)

This PR reverts https://github.com/OffchainLabs/prysm/pull/16100.

**What type of PR is this?**
Bug fix


**What does this PR do? Why is it needed?**
This PR reverts https://github.com/OffchainLabs/prysm/pull/16100 as that
PR deprecates mplex but other implementations only support mplex for
now..


**Which issues(s) does this PR fix?**

Fixes #

**Other notes for review**

**Acknowledgements**

- [x] I have read
[CONTRIBUTING.md](https://github.com/prysmaticlabs/prysm/blob/develop/CONTRIBUTING.md).
- [x] I have included a uniquely named [changelog fragment
file](https://github.com/prysmaticlabs/prysm/blob/develop/CONTRIBUTING.md#maintaining-changelogmd).
- [x] I have added a description with sufficient context for reviewers
to understand this PR.
- [x] I have tested that my changes work as expected and I added a
testing plan to the PR description (if applicable).

cmd/flags.go

@@ -224,11 +224,6 @@ var (
 		Name:  "enable-upnp",
 		Usage: "Enable the service (Beacon chain or Validator) to use UPnP when possible.",
 	}
-	// EnableAutoNATFlag enables AutoNAT v2 service for per-address reachability detection.
-	EnableAutoNATFlag = &cli.BoolFlag{
-		Name:  "enable-autonat",
-		Usage: "Enable AutoNAT v2 service for per-address reachability detection. Helps diagnose connectivity issues behind NAT/firewalls.",
-	}
 	// ConfigFileFlag specifies the filepath to load flag values.
 	ConfigFileFlag = &cli.StringFlag{
 		Name:  "config-file",