Consider dropping hand-written cipher modes

[scott-xu]

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

1. CbcCipherMode will not be used anymore once PR Use System.Security.Cryptography for TripleDesCipher #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 #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.