GtkButtonBox: remove spacing when buttons are linked

GtkButtonBox adds the "linked" class to its style context when its
layout is set to GTK_BUTTONBOX_EXPAND. It shouldn't ever make sense to
have spacing between buttons in that case, as themes generally draw
linked elements with a continuous border.

Thus, always set spacing to 0 and ignore GtkDialog's button-spacing
style property when the layout is set to EXPAND.

Also remove the now-redundant css rules which set button-spacing to 0
for message dialogs.

https://bugzilla.gnome.org/show_bug.cgi?id=752131

gtk/theme/HighContrast/_common.scss

@@ -1998,7 +1998,6 @@ GtkCalendar {
 }
 
 .message-dialog { // Message Dialog styling
-  -GtkDialog-button-spacing: 0;
   .titlebar {
     border-style: none;
     box-shadow: inset 0 1px $borders_edge;