Stop forcing icon style for sidebars

The sidebar property `-gtk-icon-style: regular` was
forcing regular icons even if apps requested
symbolic icons and the icon theme provided them.

This now allows Thunar to use a symbolic eject icon.
It also allows Nemo and Rhythmbox to use symbolic icons.

For reference, this line was previously added in #0cd4db5

Author: newhoa

light/gtk-3.0/_common.scss

@@ -3844,7 +3844,6 @@ filechooserbutton:drop(active) {
 .sidebar {
   border-style: none;
   background-color: $sidebar_bg_color;
-  -gtk-icon-style: regular;
 
   row,
   treeview {

light/gtk-4.0/_common.scss

@@ -3958,7 +3958,6 @@ filechooserbutton:drop(active) {
 .sidebar {
   border-style: none;
   background-color: $sidebar_bg_color;
-  -gtk-icon-style: regular;
 
   row,
   treeview,