REST API: Require the reassign parameter when deleting users.

When deleting a user through the WordPress admin, a specific decision is presented - whether to assign all of the user's posts to another user, or to delete all of the posts.

This change requires `reassign` as a parameter in the corresponding REST API endpoint, so that content isn't accidentally lost.

Props jeremyfelt.
Fixes #39000.


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git-svn-id: https://core.svn.wordpress.org/trunk@39366 1a063a9b-81f0-0310-95a4-ce76da25c4cd