This is a clean-up task in the context of removing the onboarding elements from the codebase.

In T340644, we added a user preference to remember when the user didn’t want to be shown the popup about mul again. Since the popup as a whole is a temporary feature (source: TODO), and we eventually intend to remove it again (task TBD), at that point we should also remove the preference from production again, to avoid bloating the table.

The preference is called wb-dont-show-again-mul-popup.

See also T342264: Remove wikibase-otherprojects from user preferences (user_properties) for a similar cleanup, possibly smaller in scale; we could do that one first, to gain familiarity with how to clean up preferences.