update CanSecond message process logic to support elastic scaling

[axaysagathiya]

Issue summary

in secondingSanityCheck method of candidateBacking struct, we have used perLeaf field of candidateBacking struct to iterate over the active leaves, Which keeps the record of whether the prospective parachain mode is enabled or disabed in active leaves.

now as we don't want to use prospective parachain mode anymore.

• instead of iterating over perLeaf, iterate over leaves in the implicit view.

• also remove code for prospective parachain mode disabled.

• Logic of this function written in the context of fragment tree, needs to update it to use fragment chain.

  • get hypothetical membership of the candidates from prospective parachain(res: candidate and list of leaves where the candidate could be added)
  • for each active leaf check if check if they are present in the list of leaves where the candidate could be added. if present it's possible to second the candidate at that leaf.
  • return the list possible of active leaves where candidate can be seconded.(return type changed)

• also secondingSanityCheck method has comment about fragment tree, update it with the context of fragment chain.

• in CanSecond method,

  • remove code related to fragment tree.
  • if secondingSanityCheck returns empty list of active leaves, it's not possible to second the candidate.

TODO:

• make sure we do not have any comment or code related to fragment tree as we want to update the code to use fragment chain

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