Remove unused skip_confirmation param from trials controller

TrialRegistrationsController is allowing skip_confirmation through as a user param

added in !15213 (merged) as part of #13233 (closed) ...since this is a backend/frontend splitting MR concept, I had to search through a few other MRs, I can't find it ever being used in the view layer/passed to the backend from the frontend. However, it was discussed in !15213 (comment 209051497) and kind of ok'd in https://gitlab.com/gitlab-com/gl-security/security-operations/trust-and-safety/operations/-/issues/200#note_572994612 and gitlab-foss#47003 (closed)

Solution

Remove the permitted skip_confirmation from TrialRegistrationsController