wlroots: drop ffmpeg, libpng dependencies

These were removed from the official CI over a year ago [1] and even
before that they were only present for the examples.

[1]: https://gitlab.freedesktop.org/wlroots/wlroots/-/merge_requests/4490

pkgs/development/libraries/wlroots/default.nix

@@ -15,8 +15,6 @@
   libcap,
   libgbm,
   xorg,
-  libpng,
-  ffmpeg,
   hwdata,
   seatd,
   vulkan-loader,
@@ -77,13 +75,11 @@ let
 
       buildInputs =
         [
-          ffmpeg
           libliftoff
           libdisplay-info
           libGL
           libcap
           libinput
-          libpng
           libxkbcommon
           libgbm
           pixman