update logic to process validated candidate command to support elastic scaling

[axaysagathiya]

Issue summary

Validated candidate command is an internal message used within the candidate backing subsystem.

There are three validated candidate commands (enum)

• No changes are needed in the logic to process Attest and AttestNoPov commands.
• there are minor changes in the logic to process Second command

Implementation details

modification in a logic to process Second command:

• Some logic seems to be not required. So remove this code where we compare
  persistedValidationData.ParentHead.Data hash with commitments.HeadData.Data hash.
• remove the code to update seconded depths in active leaves, which is related to the fragment tree.

Other information and links

Acceptance criteria