In 0d5df6f0 (repository: Implement `#new_blobs` via `#list_blobs`,
2021-08-25), we have introduced an alternative implementation of
`#new_blobs` which uses `ListBlobs()` instead of `ListNewBlobs()`. While
there wasn't yet any change in behaviour, this conversion allows us to
convert `#new_blobs` to receive multiple new revisions as arguments such
that we can batch-load new blobs in a single call instead of having to
do so once per change.

Remove the feature flag that has been guarding this new implementation
such that we can continue with the conversion. While the new code has
only been tested in production for one day at the time of writing this
commit, there haven't been any issues. Furthermore, the risk of this
change was low from the beginning. So removing the feature flag this
quickly shouldn't be much of a concern.

Changelog: added