themes: Removing padding constraints on GimpToolDialog

Resolves Issue #9955.
GimpToolDialog CSD buttons were being singled out to only
have padding of 2px. This was inconsistent with other dialogues
without this constraint. Since the larger padding style seems to
be preferred, we'll remove the constraint for GimpToolDialog
CSD buttons.

themes/System/gimp.css

@@ -74,10 +74,6 @@ GimpToolDialog headerbar {
     min-height: 0;
 }
 
-GimpToolDialog headerbar button {
-    padding: 2px 2px 2px 2px;
-}
-
 GimpDock frame {
     -GimpFrame-label-bold:       0;
     -GimpFrame-label-spacing:    2;