Remove warning about missing statement file

[wil93]

When running task-maker-rust on a task, I get this warning:

Error: Missing statement file (statement/statement.pdf or testo/testo.pdf)
 Note: Without that file cms will not be able to import the task
 Help: Try: ln -s english.pdf testo/testo.pdf

This is not true anymore since cms-dev/cms#1253 was merged, so it would make sense to either remove the warning, or to keep it but only show it if a statement/<language_name>.pdf file is missing (which would still be a useful warning).