[server test] simplify data-schemaserial test instance's schema

[multi-sources-removal] Drop (dont_)cross relation related code That was only needed for now deleted 'true' multisources. Related to #2919300 [jcr: also remove it from server/test/data-schemaserial/schema.py]

[multi-sources-removal] Simplify repo.delete_info_multi arguments, uri is no more used Related to #2919300

[multi-sources-removal] Drop deleted_entities system table and entities.mtime column since they were only used by the entities_modified_since api of the repository which has been dropped. Along with them, the multi-sources-etypes configuration variable and some sql queries at modification/deletion time of entities. Bon vent ! Related to #2919300

[multi-sources-removal] drop repository entities_modified_since public api actually only used by pyrorql/zmqrql sources which have been dropped. This will allow to drop a bunch of system meta-data (eg deleted_entities) Related to #2919300

[multi-sources-removal] Drop the "true" multi-sources planner Only keep copy-based sources. This will soon allow for greater code simplification. Related to #2919300

[multi-sources-removal] Drop cw.server.sources.extlite no more used for a while, beside an utility class Related to #2919300

[multi-sources-removal] Move cw.server.sources.ConnectionWrapper where it belongs It's only used by the ldap utilities. Related to #2919300

[multi-sources-removal] Drop pyrorql and zmqrql sources After a few years experementing "true" multi-sources, we're now moving to "copy-based" source à la datafeed. As pyro and zmq sources have no more known customers and the related code is in the way of future refactoring of cubicweb's core, we decided to drop support for those sources without backward compatibility. If you're still using a zmqrql or pyrorql source and you want to upgrade, ask support to move it to datafeed using a pre-3.19 version first. Related to #2919300 (first step)

[web/formfields] Make CompoundField inherit needs_multipart from its subfields If there's a FileField within a CompoundField, the latter will not have the enctype multipart set, even though the FileField needs it. CompoundField should inspect its children fields to determine whether it needs multipart. Closes #3406930.