style: Clean up unused -moz-window-shadow values

After bug 1768278 and bug 1767815 there's no more uses of the cliprounded value
in the tree (also it causes artifacts on HiDPI screens so we probably don't
want new usages).

The "sheet" value is unused, and the other values other than "default" and
"none" are only derived from "default", so they don't need to be exposed in the
style system.

Differential Revision: https://phabricator.services.mozilla.com/D145821

components/style/properties/longhands/ui.mako.rs

@@ -51,12 +51,15 @@ ${helpers.single_keyword(
     spec="None (Nonstandard Firefox-only property)",
 )}
 
+// TODO(emilio): Maybe make shadow behavior on macOS match Linux / Windows, and remove this
+// property.
 ${helpers.single_keyword(
     "-moz-window-shadow",
-    "default none menu tooltip sheet cliprounded",
+    "default none",
     engines="gecko",
     gecko_ffi_name="mWindowShadow",
     gecko_enum_prefix="StyleWindowShadow",
+    gecko_inexhaustive=True,
     animation_value_type="discrete",
     enabled_in="chrome",
     spec="None (Nonstandard internal property)",