Remove dead item gimptoolpresets.xml in app-docs.sgml

This item caused builds with --enable-gtk-doc-app to 
fail because of a typo and it was also misplaced in 
app-core-containers.
Remove dead item gimptoolpresets.xml in app-docs.sgml

This item caused builds with --enable-gtk-doc-app to
fail because of a typo and it was also misplaced in
app-core-containers.