Merge: Delete underlayFile attribute

Following issue #46, and as agreed on [NIDASH-DDWG call on 28th of April](https://docs.google.com/document/d/1p1eV5PvNvEz001rIckDGq9I5ez-8oafZrKokJZH8ijM/edit?usp=sharing) this pull request deletes the `nidm:underlayFile` attribute.