GtkButtonBox: remove spacing when buttons are linked

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