Remove support for the menclose's "radical" notation.

See https://groups.google.com/forum/#!topic/mozilla.dev.platform/vwAkuZIEhnY

* Introduce a new preference option to disable menclose's "radical" notation.
* Disable the notation in Nightly and when running WPT tests.
* Enable the notation in other channels together with a counter and
  deprecation warning.
* Update WPT test legacy-menclose-radical-notation.html
  - Fix test: "radical" should be equivalent to "", which is not the same as
    the default value "longdiv".
    See w3c/mathml#144
  - Add a test "box radical" which should be equivalent to "box".
  - Remove failure expectation.
* Enable the radical notation for MathML reftests testing it.

Differential Revision: https://phabricator.services.mozilla.com/D46721

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1548522
gecko-commit: d2613eaa13690e3771e84ef6ff5080a8fa348a66
gecko-integration-branch: autoland
gecko-reviewers: emilio

mathml/presentation-markup/menclose/legacy-menclose-radical-notation-ref.html

@@ -15,6 +15,9 @@
       <menclose>
         <mn>123</mn>
       </menclose>
+      <menclose notation="box">
+        <mn>123</mn>
+      </menclose>
     </math>
   </body>
 </html>

mathml/presentation-markup/menclose/legacy-menclose-radical-notation.html

@@ -5,7 +5,7 @@
     <title>Legacy menclose radical notation</title>
     <link rel="help" href="https://www.w3.org/TR/MathML3/chapter3.html#presm.menclose">
     <link rel="help" href="https://mathml-refresh.github.io/mathml-core/#enclose-expression-inside-notation-menclose">
-    <meta name="assert" content="Verify that the legacy menclose radical notation is not supported.">
+    <meta name="assert" content="Verify that the legacy menclose radical notation is ignored.">
     <link rel="match" href="legacy-menclose-radical-notation-ref.html">
     <link rel="stylesheet" type="text/css" href="/fonts/ahem.css" />
     <style>
@@ -19,6 +19,9 @@
       <menclose notation="radical">
         <mn>123</mn>
       </menclose>
+      <menclose notation="box radical">
+        <mn>123</mn>
+      </menclose>
     </math>
 
    <script src="/mathml/support/feature-detection.js"></script>