gtkcssprovider: deprecate gtk_css_provider_get_default

This function is unused since cdc6e827 commit and does not
contain fallback style.

#1226
1 job for wip/muktupavels/deprecate-gtk-css-provider-get-default in 8 minutes and 47 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  Build
passed build #67374

00:08:47