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Here is the working solution I ultimately wound up using. It is based on tarsius' advice, who deserves all the credit.

(defun kill-magit-diff-buffer-in-current-repo (&rest _)
  "Delete the magit-diff buffer related to the current repo"
  (let ((magit-diff-buffer-in-current-repo
         (magit-mode-get-buffer 'magit-diff-mode)))
    (kill-buffer magit-diff-buffer-in-current-repo)))
;;
;; When 'C-c C-c' is pressed in the magit commit message buffer,
;; delete the magit-diff buffer related to the current repo.
;;
(add-hook 'git-commit-setup-hook
          (lambda ()
            (add-hook 'with-editor-post-finish-hook
                      #'kill-magit-diff-buffer-in-current-repo
                      nil t))) ; the t is important