Comments are not properly handled in case of XWiki format translations

**Describe the issue**

Translation key comments are not properly updated when the comment changed on the source file. We saw the problem when moving a key, which led it for not having the comment anymore: apparently all translation keys keeps displaying the comment attached to it, where they should not.
We probably need to always sync the translation comment to the source comment in XWiki format.

**To Reproduce the issue**

Steps to reproduce the behavior:

1. Create a source translation with a comment in XWiki Java Properties format
2. Translate it in several languages
3. Remove the comment or update it in source translation
4. See that translation still have the comments: if the translation is edited, the comment still appear on the output file

**Expected behavior**

The comment should always be updated from the source translation comment.

**Additional context**

See the following commit that show the issue: https://github.com/xwiki/xwiki-platform/pull/1603#discussion_r617363675