gs-common: Don’t leak GtkCssProvider for custom CSS

While the old providers weren’t leaked, they were never used again and their resources were never freed, so they were effectively leaked. In particular, their resources included loaded images for GsFeatureTile backgrounds, which were quite large pixbufs.

This reduces gnome-software’s pixbuf heap consumption from 47MB to 23MB after loading. The previous consumption was so high due to refreshing featured tile CSS a few times during setup.

Signed-off-by: Philip Withnall withnall@endlessm.com