drep registration-certificate: remove mix with SPO registration

[smelc]

Changelog

- description: |
    - Remove SPO registration bits from drep registration-certificate command
    - Remove drep command from eras before Conway
    - Remove `AnyRegistrationTarget`
    - Remove `RegistrationTarget`
# uncomment types applicable to the change:
  type:
  # - feature        # introduces a new feature
  - breaking       # the API has changed in a breaking way
  # - compatible     # the API has changed but is non-breaking
  # - optimisation   # measurable performance improvements
  - improvement    # QoL changes e.g. refactoring
  # - bugfix         # fixes a defect
  # - test           # fixes/modifies tests
  # - maintenance    # not directly related to the code
  # - release        # related to a new release preparation
  # - documentation  # change in code docs, haddocks...

Context

• Contributes to fixing drep registration certificate mixed with spo registration (clean-up) #198
• Follow-up PR will be required after drep registration certificate mixed with spo registration (clean-up) #198 (comment) is tackled

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• The change log section in the PR description has been filled in
• NA New tests are added if needed and existing tests are updated. These may include:
  • golden tests
  • property tests
  • round trip tests
  • integration tests
    See Running tests for more details
• NA The version bounds in .cabal files are updated
• CI passes. See note on CI. The following CI checks are required:
  • Code is linted with hlint. See .github/workflows/check-hlint.yml to get the hlint version
  • Code is formatted with stylish-haskell. See .github/workflows/stylish-haskell.yml to get the stylish-haskell version
  • Code builds on Linux, MacOS and Windows for ghc-8.10.7 and ghc-9.2.7
• The changelog section in the PR is updated to describe the change
• Self-reviewed the diff

[smelc]

As of the current tip, the API of drep registration-certificate is as follows:

Usage: cardano-cli conway governance drep registration-certificate 
                                                                     ( --drep-verification-key STRING
                                                                     | --drep-verification-key-file FILE
                                                                     | --drep-key-hash HASH
                                                                     )
                                                                     --key-reg-deposit-amt NATURAL
                                                                     --out-file FILE

  Create a registration certificate.

Available options:
  --drep-verification-key STRING
                           DRep verification key (Bech32 or hex-encoded).
  --drep-verification-key-file FILE
                           Filepath of the DRep verification key.
  --drep-key-hash HASH     DRep verification key hash (either Bech32-encoded or
                           hex-encoded). Zero or more occurences of this option
                           is allowed.
  --key-reg-deposit-amt NATURAL
                           Key registration deposit amount.
  --out-file FILE          The output file.
  -h,--help                Show this help text

[smelc]

Follow-up PR will be required after #198 (comment) is tackled