Remove unused encrypted test keys #1003
Description
As part of #604, we added rsa, ecdsa and ed25519 test key pairs, with plain text and encrypted private keys. See tests/data/pems/*_private_encrypted.pem.
Latest pyca/cryptography version does not seem to support the (outdated) encryption algorithm anymore. See in-toto, which uses the same test keys: in-toto/in-toto#882 (comment)
This is not a problem for securesystemslib, because we don't use the encrypted keys in our tests, which makes sense, because we don't provide API for decryption.
Let's remove these 3 keys.