REST API: Disable `DELETE` requests for users in multisite.

In wp-admin, users are removed from individual sites rather than deleted. A user can only be deleted from the network admin.

Until support for a `PUT` request that removes a user's site and content associations is available, `DELETE` requests are disabled to avoid possible issues with lost content.

Props jnylen0, rachelbaker.
Fixes #38962.

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git-svn-id: https://core.svn.wordpress.org/trunk@39378 1a063a9b-81f0-0310-95a4-ce76da25c4cd