Drop FK for ci_pending_builds

Max Oreficerequested to merge
morefice/drop-fk-ci-pending-build into master

Ref: #388092 (closed), #382033 (closed)

What does this MR do and why?

Following up !109311 (merged) where we added our new FK including the partition key in order to partition the ci_builds table.

This MR removes our old FK on ci_pending_builds which is currently preventing us to partition ci_builds.

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