gnu: mpv: Use packaged waf.

* gnu/packages/video.scm (mpv)[inputs]: Remove externally downloaded waf
package and use packaged python-waf.

gnu/packages/video.scm

@@ -805,14 +805,7 @@ SVCD, DVD, 3ivx, DivX 3/4/5, WMV and H.264 movies.")
        ("pulseaudio" ,pulseaudio)
        ("rsound" ,rsound)
        ("vapoursynth" ,vapoursynth)
-       ("waf" ,(origin
-                 (method url-fetch)
-                 ;; Keep this in sync with the version in the bootstrap.py
-                 ;; script of the source tarball.
-                 (uri "http://www.freehackers.org/~tnagy/release/waf-1.8.12")
-                 (sha256
-                  (base32
-                   "12y9c352zwliw0zk9jm2lhynsjcf5jy0k1qch1c1av8hnbm2pgq1"))))
+       ("waf" ,python-waf)
        ("youtube-dl" ,youtube-dl)
        ("zlib" ,zlib)))
     (arguments