There appears to be a strange bug whereby the comment_id index can sometimes be missed
or missing from the action table despite the sync2 that should create it in the earlier
part of this migration. However, looking through the code for Sync2 there is no need
for this pre-code to exist and Sync2 should drop/create the indices as necessary.
I think therefore we should simplify the migration to simply be Sync2.
Signed-off-by: Andrew Thornton <art27@cantab.net>
