LFK: Remove foreign key requirements_management_test_reports.build_id

What does this MR do and why?

In !77908 (merged) we already added a "loose foreign key" to replace this foreign key. Now that this has been running fine in production we should be OK to remove the foreign key altogether.

This was added in a post-deployment migration as we want to ensure that they have the correct code running with the loose foreign key in place before we remove the foreign key to ensure that there isn't any lost deletes.

Migration output

Up

== 20220110231420 RemoveRequirementsManagementTestReportsBuildIdFk: migrating =
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:requirements_management_test_reports)
   -> 0.0049s
-- remove_foreign_key(:requirements_management_test_reports, :ci_builds, {:name=>"fk_rails_e67d085910"})
   -> 0.0045s
== 20220110231420 RemoveRequirementsManagementTestReportsBuildIdFk: migrated (0.0133s)

Down

== 20220110231420 RemoveRequirementsManagementTestReportsBuildIdFk: reverting =
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:requirements_management_test_reports)
   -> 0.0054s
-- transaction_open?()
   -> 0.0000s
-- execute("ALTER TABLE requirements_management_test_reports\nADD CONSTRAINT fk_rails_e67d085910\nFOREIGN KEY (build_id)\nREFERENCES ci_builds (id)\nON DELETE SET NULL\nNOT VALID;\n")
   -> 0.0047s
-- execute("SET statement_timeout TO 0")
   -> 0.0006s
-- execute("ALTER TABLE requirements_management_test_reports VALIDATE CONSTRAINT fk_rails_e67d085910;")
   -> 0.0182s
-- execute("RESET statement_timeout")
   -> 0.0007s
== 20220110231420 RemoveRequirementsManagementTestReportsBuildIdFk: reverted (0.0409s)

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Related issues

Related to #348274 (closed)