Determine a better bounded context for UserMentionBehaviour concern.

As a followup to this comment, a better bounded context is needed for the UserMentionBehaviour concern that was created due to the Sec Decomposition effort needing to separate the ApplicationRecord for the VulnerabilityUserMentions model.

Once an appropriate bounded context is decided, we can update the concern accordingly.

From !164322 (comment 2095819036), the method is not even used, so consider removing it entirely