css: stop showing theme-dependent shadow for needs-attention dot

Because DzlTab doesn't behave as correctly as GtkStackSwitcher
with regard to the needs-attention dot. Particularly, the shadow
is white for adwaita-dark but GtkStackSwitcher behaves correctly
in this case and use a dark shadow. This commit removes the
shadow totally as even if($variant=='light…) didn't work when
testing.
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