Clean up the mess around $rsrc->sqlt_deploy_callback

Clean up the mess around $rsrc->sqlt_deploy_callback

Back in f89bb832 nothingmuch introduced a hook point used to better integrate
with KiokuDB::Backend::DBI. It was properly implemented as a $rsrc instance
attribute, though somewhat undocmented.

Then in 880c075b castaway came along and incorrectly documented the attribute
as settable from a class, without it ever becoming a member of the list in
DBIC::ResultSourceProxy.

Then in 3b4e619d1 yours truly came along and (either misled by the docs, or
by drugs, or both) switched the accessor to classdata (CAG inherited) even
though rsrc-class-level operations were never a thing.

Revert part of 3b4e619d1 and fix the documentation to properly reflect the
state of affairs