Make necessary changes for alpha.6

[jtraglia]

Most of this PR is undo-ing things, which is a nice change 😄

• Electra doesn't make any changes to ExecutionPayload now.
  • So replace electra.ExecutionPayload with deneb.ExecutionPayload.
• Because we use the same payload as Deneb, we no longer need any new API types.
  • So we can delete the whole api/electra directory.
• Consolidations/withdraws/etc are no longer in the payload.
  • So we can delete the getter functions for those in:
    • VersionedSubmitBlindedBlockResponse
    • VersionedSubmitBlockRequest

Note: I've added a replace directive in go.mod to @pk910's go-eth2-client branch.
When his branch has been merged into attestantio's electra branch, delete this.