Removed dependency on OAI-PMH, the menu should be inside the OAIPMH panel

[mesemus]

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Description

This pull request fixes issue #200 - invenio-administration uses url_for for OAI-PMH server but does not depend on it. The proposed solution is just to remove the explicit menu link for the new oaipmh set. A new set can be created from within the oai listing panel the same way other objects are created: https://github.com/inveniosoftware/invenio-rdm-records/blob/master/invenio_rdm_records/administration/views/oai.py

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[kpsherva]

LGTM, waiting for confirmation if the menu item is still visible in the app-rdm after this change