Drop built-in implementation of shadows-highlights and use GEGL's
Now that GEGL 0.3.28 has been released that ships gegl:shadows-highlights by default, we should switch to using it instead of our own implementation (photos:shadows-highlights). Both implementations should be identical and photos:shadows-highlights only exists because gegl:shadows-highlights was not enabled by default in previous releases.
We should make the switch before releasing GNOME Photos 3.27.90.