gdu-restore-disk-image-dialog: Don't mark size quantity as translatable
requested to merge automerisnaranja/gnome-disk-utility:gdurestorediskimagedialog-translate-fix into main
In the string "%s when compressed", "%s" stands for the size quantity (e.g. 4.2MB), something that doesn't need to be marked for translation.
Also, make some minor code style fixes.