Revisit Duo Pro seat count fallback

Problem

In https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/12251+, a fallback for the Duo Pro seat count. The fallback logic kicks in if the License#add_on_products does not include info about the Duo Pro add-on but the License#code_suggestions_seat_count attribute returns a number greater than 0. If the fallback logic is used a warning will be logged.

Proposal

Remove the fallback since there are no logged usages of it. The logic should be simplified to return 0 if no add-on products were found for an add-on

Result

Simplified code without a fallback.