Drop ctags indexer references and hidden api using it

The ctags indexer has been stopped a long time ago and recently got dropped from the
indexer base code. As the api exposing this in the webapp is hidden, we can safely
remove them.

This also fixes build [1].

Related to T4273

[1] https://jenkins.softwareheritage.org/view/swh%20master%20(draft)/job/DWAPPS/job/tests/3265/console