Remove unused include_trials flag

What does this MR do and why?

While working on an upcoming MR in the scope of customers-gitlab-com#7380 (closed), I noticed this unused flag which was removed on this MR.

This MR removes that flag.

Screenshots or screen recordings

N/A

How to set up and validate locally

Lookup for the usage of has_a_paid_hosted_plan? and verify the flag is not used anymore.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.