Remove unused ci_stages FK

Ref: #436528 (closed)

What does this MR do and why?

This MR removes an unused FK on ci_stages now that we added a FK including partition_id in #436526 (closed).

Why are we doing this?

Necessary step in order to swap the primary key for ci_stages and make it a composite FK.

https://docs.gitlab.com/ee/development/database/partitioning/list.html#step-5---swap-primary-key