Omit the default `repository_url` from Maven PURLs

PURLs of "maven" type have an [implicit default](https://github.com/package-url/purl-spec/blob/master/PURL-TYPES.rst#maven) `repository_url` of https://repo.maven.apache.org/maven2. Currently, the `repository_url` is always specified even if it matches the default. For readability, I'd propose to only specify it if it points to a non-default repository.