treeview: don't force a border-radius to zero
treeview: don't force a border-radius to zero

This also sets border-radius to zero for all the cell renderers, which
is not something we want.

Related to: https://bugzilla.gnome.org/show_bug.cgi?id=673321

rgcjonas/adwaita-semidark,ChrisCummins/gnome-themes-standard,cosimoc/gnome-themes-standard,rgcjonas/adwaita-semidark,magcius/gnome-themes-standard,ChrisCummins/gnome-themes-standard,ChrisCummins/gnome-themes-standard,rgcjonas/adwaita-semidark,magcius/gnome-themes-standard,cosimoc/gnome-themes-standard,DoctorJellyface/gnome-themes-standard,ChrisCummins/gnome-themes-standard,cosimoc/gnome-themes-standard,rgcjonas/adwaita-semidark,ChrisCummins/gnome-themes-standard,Distrotech/gnome-themes-standard,Distrotech/gnome-themes-standard,DoctorJellyface/gnome-themes-standard,magcius/gnome-themes-standard,cosimoc/gnome-themes-standard