rm deprecated grandfather 'cel-gaulish'

test/intl402/Locale/getters-grandfathered.js

@@ -25,13 +25,6 @@ info: |
 features: [Intl.Locale]
 ---*/
 
-// Regular grandfathered language tag.
-var loc = new Intl.Locale("cel-gaulish");
-assert.sameValue(loc.baseName, "cel-gaulish"); // Step 5.
-assert.sameValue(loc.language, "cel-gaulish");
-assert.sameValue(loc.script, undefined);
-assert.sameValue(loc.region, undefined);
-
 // Regular grandfathered language tag.
 assert.throws(RangeError, () => new Intl.Locale("zh-min"));