[EFM] Remove backward-compatibility for flow.DKGIndexMap

[durkmurder]

Context

During Mainnet26, we plan to deploy a change to the EpochCommit data structure, implemented in EFM workstream. In #6785 we introduce a way to handle new changes in a backward-compatible manner:

• The field is nil-able, and instances where the value is nil are interpreted as the old data model
• Business logic operating on the data structure have 2 codepaths: one for the old data model (nil field), one for the new data model (non-nil field).

Backward-compatibility is necessary to support a downtime-free upgrade, however we do not want to maintain this extra logic longer than necessary. This issue is to remove extra logic needed for backward-compatibility.

Definition of Done

• Follow TODO(EFM, #6794) to find all places were logic was modified to support backward-compatibility and remove/update them.
• Require EpochCommit.DKGIndexMap != nil