[SM Cloud Licensing Trials] Clean up order, subscription and trial lookup logic from CreateSubscriptionSeatLinkService and CreateTrialSeatLinkService

Problem

With the introduction of CreateSeatLinkService in https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/13272 to handle the switch between CreateSubscriptionSeatLinkService and CreateTrialSeatLinkService, the order, subscription and trial lookup logic that is normally handled in those child services had also been duplicated into CreateTrialSeatLinkService

The end goal is to have this lookup logic centralized only within CreateSeatLinkService, and to then pass the found subscription or trial as parameters to CreateSubscriptionSeatLinkService or CreateTrialSeatLinkService, respectively

However, we will need to wait until all usages of CreateSubscriptionSeatLinkService have been migrated to CreateSeatLinkService first before we can update the parameters

Proposal

TBD - Will update this portion of the description once https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/13272 has been fully reviewed and merged