Remove legacy keypair support

[paulhauner]

Description

#1138 introduces a new key management system, focused around encrypted keystores. We have had to maintain support for our old, unencrypted SSZ-encoded predecessor for now, but we definitely shouldn't in the long term.

Once the current testnets (e.g., Schlesi) have finished and everyone has generated their keys under the new scheme we can just delete this code from Lighthouse. I've marked the issues as blocked until this is the case.

How to remove

This code is behind the unencrypted_keys feature of the validator_dir. To remove the code:

• Delete all functionality behind the unencrypted_keys feature (and the feature itself)
• Delete anything downstream that was relying upon that code.

[paulhauner]

This has already been done.