[Suggestion] Remove redundant `isDescribed` annotations from OMEX metadata/RDF files

[jonrkarr]

For example, 100596964 has two annotations for alternative ids for the same paper. To a user, especially without a human-readable label for the PubMed citation, this will look like citations for two papers rather than two ids for the same paper.

We prefer DOIs over PubMed ids since DOIs have a broader scope. I suggest removing the redundant PubMed annotations.

<bqmodel:isDescribedBy>
    <rdf:Description>
        <dc:identifier rdf:resource="http://identifiers.org/pubmed:27783155"/>
        <rdfs:label>pubmed:27783155</rdfs:label>
    </rdf:Description>
</bqmodel:isDescribedBy>

<bqmodel:isDescribedBy>
    <rdf:Description>
	<dc:identifier rdf:resource="http://identifiers.org/doi:10.1007/s00424-016-1893-7"/>
	<rdfs:label>Pflugers Arch. 2016 Nov;468(11-12):2031-2040.</rdfs:label>
    </rdf:Description>
</bqmodel:isDescribedBy>