move checksum and tempfile delete to delete worker

This removes two DB calls from the latency-critical queryworker
process.  This may widen a race condition with reused explicit
FIDs, but explicit FIDs are a bad idea anyways and reusing
FIDs likely had problems before this change.

I've also removed the Postgres-specific delete_fidid()
function.  commit 7dbfb44d4f443bc5a9c30d772f3842f564efb714
(Make postgres use new delete worker code) removed the
Postgres-specific code path and made it functionally
identical to the generic version.