Fixes <https://bugs.gnu.org/27556>.
* gnu/packages/image.scm (libpng-apng): Update to 1.6.28.
Remove inherit of 'libpng'.
[version]: Use own version, remove 'package-version libpng'.
[source]: Don't inherit the source of libpng.
[arguments]: Update hash of libpng-apng source.
Signed-off-by: Leo Famulari <leo@famulari.name>