Consider dropping hand-written cipher modes

We could consider dropping hand-written cipher modes in the library.

1. CbcCipherMode will not be used anymore once PR https://github.com/sshnet/SSH.NET/pull/1546 is merged.
2. CfbCipherMode is only used for PKCS1 private key encrypted byDES-EDE3-CFB. Suggest user switching to newer encryption method.
3. OfbCipherMode is not used anywhere inside the library.
4. CtrCipherMode is used for AesCipher. We can create a dedicated class AesCtrCipher, just like AesGcmCipher.

With all above concrete classes removed, we can remove the abstract class CipherMode. We can also remove AesCipherMode enum (renamed as BlockCipherMode in PR https://github.com/sshnet/SSH.NET/pull/1546) and use BCL's CipherMode enum.

Then AesCipher.BlockImpl can be removed, AesCipher.CtrImpl can be removed (moved to a dedicated class per above suggestion).
AesCipher.BclImpl can be removed so that AesCipher can have a pure BCL implementation (without nested Impl classes)

So does the TripleDesCipher.

Review PR 1546 first to confirm which cipher-mode changes it already covers. Then trace the usages of CbcCipherMode, CfbCipherMode, OfbCipherMode, CtrCipherMode, CipherMode, AesCipher and TripleDesCipher, including the PKCS1 private-key path. Done means unused hand-written modes and obsolete implementations or enums are removed without leaving required AES-CTR or TripleDES behavior unsupported.