Remove glossary item translations when marking item as non-translatable

### Describe the problem

I'm cleaning up our glossary, and there are a few strings which are brand names, which should not be translated. The terms have been added by contributors, and I'm marking them as untranslatable. However, some translators have already translated them.

For example: https://hosted.weblate.org/translate/antennapod/glossary/en/?sort_by=source&offset=11#translations

### Describe the solution you would like

When marking a term as untranslatable, also delete its translations.

### Describe alternatives you have considered

_No response_

### Screenshots


### Additional context

Related: #14560