remove the conformance class requirement from the subset module.

[joanma747]

It should not be here. Actually the other modules does not have it and the document does not include it.

https://github.com/opengeospatial/ogcapi-common/blob/master/api_modules/subset/requirements/REQ_subset_conformance.adoc

[jerstlouis]

Related to #337 simplification / clean-up , discuss whether we still need to keep this:

https://github.com/opengeospatial/ogcapi-common/tree/master/api_modules

directory.

Possibly we can re-organize the OpenAPI portions of it into an openapi/ directory at the root as in Maps.

Should we merge collections/openapi separate from 19-072/openapi (Core) at the root?

This facilitates bundling an example OpenAPI definition that includes both Part 1 and Part 2 (like we did in Processes).