Remove ChunkBody backward-compatibility

[jordanschalm]

Context

During Mainnet26, we plan to deploy a change to the ChunkBody data structure, implemented in #6744. This introduces a new field 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.

Hints

Changes to remove are implemented in #6744 and #6796. They are annotated with //TODO(mainnet27, #6773).

Definition of Done

• Make ServiceEventCount field non-nilable
• Remove business logic handling nil field case (eg. ExecutionResult.ServiceEventsByChunk)
• Remove ID and encoding compatibility test cases (example)