gnu: Further simplify package inputs.

This is the result of running:

  ./pre-inst-env guix style --input-simplification=safe

and manually undoing changes on a dozen of packages to reduce
rebuilds (derivations for emacs, icecat, and libreoffice are unchanged.)

gnu/packages/graphics.scm

@@ -138,9 +138,7 @@
     (native-inputs
      (list luajit pkg-config))
     (inputs
-     `(("alsa" ,alsa-lib)
-       ("sdl" ,sdl)
-       ("sdl2" ,sdl2)))
+     (list alsa-lib sdl sdl2))
     (synopsis "Memory Mapped Machine")
     (description "MMM is a shared memory protocol for virtualising access to
 framebuffer graphics, audio output and input event.")