Remove docs for partitioned_foreign_keys

This table was dropped in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/82215

db/docs/partitioned_foreign_keys.yml

----
-table_name: partitioned_foreign_keys
-classes: []
-feature_categories:
-- database
-description: TODO
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/commit/9af97ee69a36de1dc4e73f4030d6316d3f0a82c5
-milestone: '13.0'