MAINTAINERS: Remove usage of manifest-<module> GitHub labels

The manifest-<module> GitHub labels are automatically added by the
manifest workflow whenever a PR modifies the revision of a project in
the manifest file. This allows users to filter by PRs that modify
revisions of particular projects, which is a useful feature.

The MAINTAINERS.yml file had assigned manifest-<module> labels to the
areas corresponding to west projects, which means that Pull Requests
that modify glue code or otherwise code in the main repository that is
assigned to that entry (mostly code in modules/) currently get labeled
with the manifest-<module> label. Fix this by not using those labels in
the MAINTAINERS file anymore.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>