Remove `Spec::Support::Helpers::KeyGeneratorHelper` class

What does this MR do and why?

Spec::Support::Helpers::KeyGeneratorHelper was added in 857d0391 to generate RSA public key in openssh format. At that time we didn't have a gem in Gemfile to do so.

Later, we added sshkey gem that allows generating public keys.

Also, we used to have SSHKeygen class to generate RSA public keys in openssh format for specs:

In !77424 (merged), !78532 (merged) we started using ssh_data gem instead of net-ssh, sshkey, net-ssh, ed25519, bcrypt_pbkdf, and SSHKeygen-class.

We can remove Spec::Support::Helpers::KeyGeneratorHelper in favor of ssh_data gem too.

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