Repo cleanup

Let's clear things up just before #8 (closed):

• The attempt to use OpenAPI in meta/ is removed;
• transkribus.dump is removed; it used a hardcoded collection ID, was never used anywhere, and even its tests are redundant;
• A method and a property from transkribus.xml.XmlElement are removed as they are not used in this packge or in our repos, nor are they covered in any test.

This accidentally bumps the coverage to 96%, not bad.