Remove Openssh checkpoint from hardware key test plan

[Joerger]

Removes OpenSSH checkpoint as it has never worked.

For context: during hardware key agent testing, I mistakenly thought it solved the issue with Hardware Key support and the OpenSSH client described here. What actually changed is that tsh proxy ssh is now able to prompt for PIN through the agent whereas before you would get the following error:

> ssh server01.root.example.com 
Enter your YubiKey PIV PIN:
ERROR: failed to perform warmup signature with hardware private key
	pin cannot be empty

Connection closed by UNKNOWN port 65535

If you run the hardware key agent, you can now get one step further, entering the PIN through the agent, but ultimately failing the key exchange:

> ssh server01.root.example.com
Load key "/Users/bjoerger/.tsh/keys/root.example.com/dev-ssh/root.example.com-cert.pub": error in libcrypto
bjoerger@server01.root.example.com: Permission denied (publickey).

As described in #39339 (comment), we still need a way to inject a teleport client aware of hardware keys into the key exchange. SSH Vnet has been chosen for that job as an alternative.