Revert "Merge branch 'mk-create_work_item_description_table' into 'master'"

What does this MR do and why?

Revert "Merge branch 'mk-create_work_item_description_table' into 'master'"

This reverts merge request !201103 (merged)

I found that partitioning by root_namespace_id causes problems when syncing the data, see !202063 (comment 2739172953) for details.

The best approach should be therefore:

1. In this MR: Revert this MR within 18.4 so the work_item_descriptions table never gets created on self managed
2. In this MR: add a post_migration to delete the table (it's empty)
3. Next MR: Re-create the previous MR but this time partitioned by namespace_id

The table is empty on .com because we have not written to it yet. It has also never been deployed in a release.

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