incremental export should be in same format as full export
This ticket is for the katello changes associated with https://pulp.plan.io/issues/1543.
Once Pulp supports incremental exports using the same format as the full export (i.e., with repodata), Katello should also support importing/exporting via the same means.
Fixes #14915: make use of Pulp's incremental repomd feature
Newer versions of Pulp have a `incremental_repomd` flag. This allows
incremental exports to be in the same format as "regular" exports.
This patch allows Katello to use this new flag. It also removes
Katello's special handling of incremental imports which was very slow
and is no longer needed.