Eradicate AVIF_ENABLE_EXPERIMENTAL_GAIN_MAP

Verified with the command:
  find . -name "*" -a -type f | xargs grep AVIF_ENABLE_ | \
    grep GAIN_MAP | grep -v JPEG

.github/workflows/ci-unix-shared-installed.yml

@@ -103,8 +103,7 @@ jobs:
           -DAVIF_ENABLE_NODISCARD=ON -DAVIF_ENABLE_WERROR=ON
           -DAVIF_BUILD_EXAMPLES=ON -DAVIF_BUILD_APPS=ON
           -DAVIF_BUILD_TESTS=ON -DAVIF_ENABLE_GTEST=ON -DAVIF_GTEST=LOCAL
-          -DAVIF_BUILD_GDK_PIXBUF=ON  ${{ env.CMAKE_AVIF_FLAGS }}
-          -DAVIF_ENABLE_EXPERIMENTAL_GAIN_MAP=ON
+          -DAVIF_BUILD_GDK_PIXBUF=ON ${{ env.CMAKE_AVIF_FLAGS }}
       - name: Build libavif with [[nodiscard]] (ninja)
         if: runner.oldest-cmake == 'false'
         working-directory: ./build_nodiscard

tests/data/README.md

@@ -643,7 +643,7 @@ libavif and ignored by the tests.
 License: [same as libavif](https://github.com/AOMediaCodec/libavif/blob/main/LICENSE)
 
 Source : created from `seine_sdr_gainmap_srgb.jpg` converted to avif with avifenc built
-with AVIF_ENABLE_GAIN_MAP and AVIF_LIBXML2 enabled, then:
+with AVIF_LIBXML2 enabled, then:
 
 ```
 # from libavif/mybuilddir/