vocabularies: switch to datastreams for affiliations

[tmorrell]

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Description

This removes the fixtured affiliations vocabulary now that datastreams are available as well as the outdated ror yaml file. To be merged only when inveniosoftware/invenio-vocabularies#327 and inveniosoftware/invenio-app-rdm#2680 are released.

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[ptamarit]

The fact of having the download the ROR file manually is documented in inveniosoftware/docs-invenio-rdm#639, so removing the default file from the cookiecutter is OK.

Maybe it could be nice to include a link to https://inveniordm.docs.cern.ch/customize/vocabularies/ (similarly to what is done in invenio.cfg), not sure...