Remove no-longer existing API from the migration guide
Remove no-longer existing API from the migration guide

Some parts of the migration guide were written before the demise
of pixmaps, and still referred to pixmap API in their replacements.

Sidnioulz/SandboxGtk,davidt/gtk,grubersjoe/adwaita,alexlarsson/gtk,davidgumberg/gtk,Lyude/gtk-,alexlarsson/gtk,alexlarsson/gtk,simokivimaki/gtk,jigpu/gtk,Distrotech/gtk2,davidt/gtk,Lyude/gtk-,Adamovskiy/gtk,alexlarsson/gtk,jessevdk/gtk,davidgumberg/gtk,Adamovskiy/gtk,msteinert/gtk,jadahl/gtk,davidgumberg/gtk,ebassi/gtk,jadahl/gtk,davidt/gtk,Adamovskiy/gtk,jigpu/gtk,jadahl/gtk,bratsche/gtk-,msteinert/gtk,bratsche/gtk-,bratsche/gtk-,ebassi/gtk,jessevdk/gtk,ebassi/gtk,Sidnioulz/SandboxGtk,alexlarsson/gtk,jigpu/gtk,Lyude/gtk-,chergert/gtk,chergert/gtk,ebassi/gtk,jadahl/gtk,jessevdk/gtk,Adamovskiy/gtk,grubersjoe/adwaita,bratsche/gtk-,bratsche/gtk-,Lyude/gtk-,ahodesuka/gtk,msteinert/gtk,davidgumberg/gtk,jessevdk/gtk,grubersjoe/adwaita,davidt/gtk,msteinert/gtk,Lyude/gtk-,chergert/gtk,Distrotech/gtk2,jigpu/gtk,msteinert/gtk,grubersjoe/adwaita,Lyude/gtk-,jessevdk/gtk,ahodesuka/gtk,ebassi/gtk,Adamovskiy/gtk,simokivimaki/gtk,Sidnioulz/SandboxGtk,davidt/gtk,Distrotech/gtk2,jadahl/gtk,jigpu/gtk,jadahl/gtk,jigpu/gtk,bratsche/gtk-,Adamovskiy/gtk,alexlarsson/gtk,alexlarsson/gtk,simokivimaki/gtk,Lyude/gtk-,jessevdk/gtk,jigpu/gtk,simokivimaki/gtk,grubersjoe/adwaita,jadahl/gtk,jessevdk/gtk,Sidnioulz/SandboxGtk,alexlarsson/gtk,ahodesuka/gtk,Sidnioulz/SandboxGtk,grubersjoe/adwaita,Lyude/gtk-,ahodesuka/gtk,Distrotech/gtk2,davidgumberg/gtk,davidgumberg/gtk,ahodesuka/gtk,jigpu/gtk,Distrotech/gtk2,grubersjoe/adwaita,chergert/gtk,simokivimaki/gtk,Distrotech/gtk2,davidt/gtk,davidgumberg/gtk,ahodesuka/gtk,chergert/gtk,Adamovskiy/gtk,Adamovskiy/gtk,chergert/gtk,ahodesuka/gtk,jadahl/gtk,chergert/gtk,davidgumberg/gtk,msteinert/gtk,ebassi/gtk,Sidnioulz/SandboxGtk,simokivimaki/gtk,ahodesuka/gtk,grubersjoe/adwaita,chergert/gtk