Remove Sky tests that we don't intend to port to the new world

sky/tests/custom-elements/registration.sky

Index: sky/tests/custom-elements/registration.sky
diff --git a/sky/tests/custom-elements/registration.sky b/sky/tests/custom-elements/registration.sky
deleted file mode 100644
index c7f7af58ad86cabbf934a9e6901b38ffd8d0f85b..0000000000000000000000000000000000000000
--- a/sky/tests/custom-elements/registration.sky
+++ /dev/null
@@ -1,27 +0,0 @@
-<import src="../resources/dump-as-text.sky" />
-<script>
-import "dart:sky";
-
-class FooElement extends Element {
-  final String tagName = "foo";
-
-  attachedCallback() {
-    print("PASS: FooElement attached.");
-  }
-
-  detachedCallback() {
-  }
-
-  attributeChangedCallbackCount(String name, String oldValue, String newValue) {
-  }
-}
-
-void main() {
-  document.registerElement("foo", FooElement);
-}
-</script>
-<foo />
-<script>
-// This script element creates a microtask checkpoint and ensures that
-// the <foo /> attached callback runs before the test ends.
-</script>