servo: Merge #20016 - Remove reference to the Gecko pref svg.paint-order.enabled (from jfkthame:master); r=bholley

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This pref (to potentially disable support for a CSS property) is to be removed from Gecko, so we should also remove the reference to it in the Servo (stylo) code; the property will be always-supported, just like the majority of others.

---
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).
        (See gecko bug https://bugzilla.mozilla.org/show_bug.cgi?id=1437267.)

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because ... it's just removing support for a pref that is never actually used (it has defaulted to true ever since it was introduced in Gecko).

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Source-Repo: https://github.com/servo/servo
Source-Revision: aacb9085e566bb4a25e3c882fad3110d862fc13d

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : bb1f672f27713c519f31f1713d6e94a5ccd8e1e2

servo/components/style/properties/longhand/inherited_svg.mako.rs

@@ -133,7 +133,6 @@ ${helpers.predefined_type("marker-end", "UrlOrNone", "Either::Second(None_)",
 ${helpers.predefined_type("paint-order", "SVGPaintOrder", "computed::SVGPaintOrder::normal()",
                           products="gecko",
                           animation_value_type="discrete",
-                          gecko_pref="svg.paint-order.enabled",
                           spec="https://www.w3.org/TR/SVG2/painting.html#PaintOrder")}
 
 ${helpers.predefined_type("-moz-context-properties",