window-list: Remove outdated style

A long time ago, the window list used to embed the bottom message
tray, which caused notifications to inherit the window-list's
font style.

Since that's no longer the case, we have no business in messing
with notification styling, so stop doing that.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/338>
(cherry picked from commit f1671bc206)

extensions/window-list/stylesheet-dark.css

@@ -81,7 +81,3 @@
   width: 24px;
   height: 24px;
 }
-
-.notification {
-  font-weight: normal;
-}