Bug 1722488 - Don't alias -apple-system to system-ui for now, to keep /css/css-fonts/animations/system-fonts.html happy.

We probably want to do this when they do something different, but for
now behavior should be the same and it causes some subtests to fail
because `getComputedStyle(..).fontFamily` for system fonts seems to
return -apple-system, but `.style.fontFamily = "-apple-system"` returns
`system-ui`.

MANUAL PUSH: Orange fix on a CLOSED TREE

servo/components/style/values/computed/font.rs

@@ -411,7 +411,7 @@ pub enum GenericFontFamily {
     Monospace,
     Cursive,
     Fantasy,
-    #[parse(aliases = "-apple-system", condition = "system_ui_enabled")]
+    #[parse(condition = "system_ui_enabled")]
     SystemUi,
     /// An internal value for emoji font selection.
     #[css(skip)]