Currently __live_restore_can_service_read will return a WT_LIVE_RESTORE_SERVICE_STATE that is FULL, PARTIAL or NONE, however since wiredtiger reads/writes are always the size of one page, if we see a PARTIAL read that means:
1. Background migration for the page is in progress.
2. There're no writes on the page yet, otherwise the whole page should have been set to -1 in bitmap and FULL will be returned by __live_restore_can_service_read.
In summary, the result of a PARTIAL read is always identical to the result of a NONE read, and this ticket is to remove the PARTIAL scenario, and tidy up the function if necessary.