Fix example modifiers (#3383)

Author: jcollins-g

testing/test_package_experiments/lib/class_modifiers.dart

@@ -18,7 +18,4 @@ base mixin class K {}
 abstract mixin class L {}
 abstract base mixin class M {}
 mixin N {}
-base mixin O {}
-interface mixin P {}
-final mixin Q {}
-sealed mixin R {}
+base mixin O {}