Drop support for https://danbooru.donmai.us/cache/tags.json. This was a
nightly dump of the tags table that was originally added in #1012. It
was never documented and never really used except for by the DanbooruUp
extension.
Setting the statement timeout at the beginning didn't work because
`PostPruner.new.prune!` clobbers the timeout (it calls `without_timeout`,
which doesn't restore the timeout properly if the timeout was zero).