Avoid using prospective parachain mode while updating active leaves

[axaysagathiya]

Issue summary

In a method to update the active leaves

• We don't need ProspectiveParachainMode anymore. So remove the use of it.
  • Remove code of ProspectiveParachainMode Disabled
  • keep the code of ProspectiveParachainMode Enabled

Also, the perLeaf field of the Candidatebacking struct will be removed in the future as it keeps track of which relay parent has ProspectiveParachainMode Enabled. So avoid using this field also.

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