As noted in #470, the anonymisation feature may have been useful, but
has not been used.

Although we could keep the feature in, it is regularly _not_ being
implemented when new repositories are being created, so a user expecting
the feature to be available will be lulled into a false sense of
security.

Instead, we should remove the feature.

In `repositorymetadata` we need to still retain a single query, as
`sqlc` requires at least one query per schema.

Closes #470, #471, #306, #283, #274, #221, #165.