Add the ability to remove a translation in a multilingual field

[monsieurtanuki]

For some reason, there may be a translation that has nothing to do here (e.g. a product name translated in English, but it was a typo as the language has no translation in English and the user meant a translation in another language).

We should be able to remove it

• for consistency (e.g. no corrupted English product name translation here)
• and for side effects (e.g. the English product name taking precedency, it becomes the de facto translation for other languages).

We cannot remove a translation today: if we put an empty string the translation becomes a "populated" empty string.
I also tried to put explicitly a null value instead of an empty string, but got rejected by /cgi/product_jqm2.pl: "Could not save product - null".

The problem should be dealt with on the server side, cf. openfoodfacts/openfoodfacts-server#8367