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.
Merges [34938] onto 4.7 branch.
Props jnylen0, rachelbaker.
Fixes#38962 for 4.7.
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