style: Make -moz-box-layout: flex default, and clean-up CSS

Now it's on by default everywhere, so all this is not needed. No behavior
change effectively since the xul.css bits being removed effectively achieve the
same.

Differential Revision: https://phabricator.services.mozilla.com/D170944

components/style/gecko/media_features.rs

@@ -618,7 +618,7 @@ macro_rules! bool_pref_feature {
 /// to support new types in these entries and (2) ensuring that either
 /// nsPresContext::MediaFeatureValuesChanged is called when the value that
 /// would be returned by the evaluator function could change.
-pub static MEDIA_FEATURES: [QueryFeatureDescription; 64] = [
+pub static MEDIA_FEATURES: [QueryFeatureDescription; 63] = [
     feature!(
         atom!("width"),
         AllowsRanges::Yes,
@@ -916,10 +916,6 @@ pub static MEDIA_FEATURES: [QueryFeatureDescription; 64] = [
     ),
     lnf_int_feature!(atom!("-moz-system-dark-theme"), SystemUsesDarkTheme),
     lnf_int_feature!(atom!("-moz-panel-animations"), PanelAnimations),
-    bool_pref_feature!(
-        atom!("-moz-box-flexbox-emulation"),
-        "layout.css.moz-box-flexbox-emulation.enabled"
-    ),
     // media query for MathML Core's implementation of maction/semantics
     bool_pref_feature!(
         atom!("-moz-mathml-core-maction-and-semantics"),

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

@@ -47,7 +47,7 @@ ${helpers.single_keyword(
 
 ${helpers.single_keyword(
     "-moz-box-layout",
-    "legacy flex",
+    "flex legacy",
     engines="gecko",
     gecko_enum_prefix="StyleMozBoxLayout",
     animation_value_type="none",