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
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 firstname.lastname@example.org