fix(themes): remove dropdown-close backdrop in Nord wide view (#8781)

Nord was the only shipped theme applying a backdrop-filter to
.dropdown-close outside the narrow-viewport @media block, dimming the
page behind any open dropdown (Settings, mark-read menu, etc.) in wide
view.

Other themes leave the click-catcher transparent in wide view and only
apply a scrim in narrow view, where it doubles as the visual backdrop
for the slide-out aside. The wide-view darken in Nord read as
inconsistent against its own narrow-view treatment (grayscale + blur)
and against every other theme.

Click-outside-to-close still works; only the visual backdrop is removed.

Co-authored-by: Bjørn A. Andersen <polybjorn@users.noreply.github.com>

p/themes/Nord/nord.css

@@ -415,10 +415,6 @@ svg:hover {
 	text-decoration: none;
 }
 
-.dropdown-close {
-	backdrop-filter: grayscale(25%) brightness(0.9);
-}
-
 .dropdown-close a:hover {
 	background: none;
 }

p/themes/Nord/nord.rtl.css

@@ -415,10 +415,6 @@ svg:hover {
 	text-decoration: none;
 }
 
-.dropdown-close {
-	backdrop-filter: grayscale(25%) brightness(0.9);
-}
-
 .dropdown-close a:hover {
 	background: none;
 }