Revert the block parts of the compaction view

This reverts commits 4b9b226 and 56129a5. A compaction sends its view
again as one input part, followed by the task and, on a retry, the note.

The ChatGPT backend writes a cache entry only at the end of the latest
message of a request, and a later request reads it only at one of its
own message ends. Parts inside a message are not lookup boundaries, so
the blocks gave no cache hit. The entry of a compaction ends after its
task, which no later compaction begins with, so first tries miss with
any split of the view. A warm-up call per prefix would write a usable
entry, but it costs more than the misses, so the first-try misses stay.

Issue: 8c5fd2a