doom: remove magit-nix3

It was predictably causing too much slowdown during magit-status (why
couldn't it do the thing in the background?!)

modules/emacs/doom/config.org

@@ -195,12 +195,6 @@ Tangle ~packages.el~ from within org-mode.
 
 (package! nix3
   :recipe (:type git :host github :repo "emacs-twist/nix3.el"))
-
-(package! magit-nix3
-  :recipe (:type git
-           :host github
-           :repo "emacs-twist/nix3.el"
-           :files ("extra/magit-nix3.el")))
 #+end_src
 * Modus-themes
 Customize Modus-themes a little to make it more peppy and vibrant.
@@ -946,11 +940,6 @@ All the modifications that are necessary enough to exist, but not big enough to
   (add-to-list 'auto-mode-alist (cons "/flake\\.lock\\'" 'json-ts-mode))
   :config
   (add-hook 'nix-ts-mode-hook #'eglot-ensure'append))
-
-(use-package! magit-nix3
-  :after magit-status
-  :config
-  (magit-nix3-flake-mode t))
 #+end_src
 
 ** EditorConfig