Stop generating po-properties for gtk

Hey,

with the upcoming release GTK 4 will no longer have meta information in 
GObject properties:

https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/4717

I have created a merge request that removes the 'properties' argument 
from the `make-pot` script and deletes all the now unused po-files in 
po-properties:

https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/4718

I guess d-l has to stop calling `./make-pot properties` somewhere?

Thanks for your help in advance!

Best,
Sophie