ffmpeg: Remove libpng dependency.

Ffmpeg doesnt have a libpng dependency. All thats needed for png support is zlib which has its own option already

Author: Sibras

ports/ffmpeg/portfile.cmake

@@ -345,10 +345,6 @@ else()
     set(OPTIONS "${OPTIONS} --disable-lzma")
 endif()
 
-if("libpng" IN_LIST FEATURES)
-    set(OPTIONS "${OPTIONS} --enable-decoder=png --enable-encoder=png")
-endif()
-
 if("mp3lame" IN_LIST FEATURES)
     set(OPTIONS "${OPTIONS} --enable-libmp3lame")
 else()

ports/ffmpeg/vcpkg.json

@@ -41,7 +41,6 @@
             "bzip2",
             "freetype",
             "iconv",
-            "libpng",
             "lzma",
             "mp3lame",
             "openjpeg",
@@ -443,19 +442,6 @@
         "libilbc"
       ]
     },
-    "libpng": {
-      "description": "PNG de/encoding via libpng",
-      "dependencies": [
-        {
-          "name": "ffmpeg",
-          "default-features": false,
-          "features": [
-            "zlib"
-          ]
-        },
-        "libpng"
-      ]
-    },
     "lzma": {
       "description": "lzma support",
       "dependencies": [