build/linux: Do not plug with useless ffmpeg content snap

libheif from GNOME SDK Snap can't directly use codecs from ffmpeg due
to the use of plugin system (plugins on LIBHEIF_PLUGIN_PATH) at build time.

Also, even if it could (by custom building, which I don't plan to do),
codecs like libde265/x265 are always present in GNOME runtime Snap
(unlike GNOME runtime flatpak) so there is no way of not installing them.

(cherry picked from commit 2e5ac18d78)

build/linux/snap/snapcraft.yaml

@@ -73,10 +73,6 @@ plugs:
     interface: content
     target: $SNAP/gnome-platform
     default-provider: gnome-46-2404
-  ffmpeg-2404:
-    interface: content
-    target: ffmpeg-platform
-    default-provider: ffmpeg-2404
 
 parts:
   buildenv:
@@ -144,8 +140,6 @@ parts:
       - -Dcheck-update=no
       - -Denable-default-bin=enabled
       - -Dbuild-id=org.gimp.GIMP.snapcraft.experimental
-    build-snaps:
-      - ffmpeg-2404
     build-packages:
       - bison
       - flex