stack/nxdtest: Remove default jinja2-template:url

This default prevented extending the section with `inline` option
because `inline` and `url` conflict. This was not a problem when
the deprecated `template` or `template = inline:` option was used.

Setting `url = < nothing >` does not resolve the conflict currently.

Since all section that extend `jinja2-template` set their own `url`
or `inline`, the default value was never used anyway.

The default `filename` option is also no longer needed.