ActorMigration: Remove possibility of read-both
When this was originally written, the plan was to read both the old and
new fields during the transition period, while stopping writes to them
midway through. It turns out that the WHERE conditions to do read-both
correctly are generally not handled well by the database and working
around that would require a lot of complicated code (see what's being
removed from ApiQueryUserContribs here, for example).
We can simplify things greatly by instead having it write both fields
during the transition period, reading from the old for the first part
and the new for the second part, as is being done for MCR.