Remove support for disk configuration source

In gitlab#217912 (closed) we introduced domain-config-source configuration flag to allow users to choose which domain source to use between disk and api.

In %14.0 we plan to remove support for old disk configuration source.

The purpose of this issue is to remove domain-source, some code may be hard to remove in one MR, so this can be extracted to follow-up issues.

Omnibus will need to be updated to remove this flag

If users can't use the disk config source, we can gradually get rid of old code supporting it.