ContextualDynamicContextTest follow up

This is a minor side issue which arose at the tail end of #1816764: Add missing type hinting to Contextual module docblocks

Normally with these bug/issues the documentation no longer described the actual implementation.

This one is in reverse...

here is a code cut and past from https://www.drupal.org/node/1816764#comment-9703923

The functions current returns null.

+++ b/core/modules/contextual/src/Tests/ContextualDynamicContextTest.php
@@ -134,8 +137,6 @@ function testDifferentPermissions() {
-   *
-   * @return bool
    */
   protected function assertContextualLinkPlaceHolder($id) {
@@ -146,8 +147,6 @@ protected function assertContextualLinkPlaceHolder($id) {
-   *
-   * @return bool
    */
   protected function assertNoContextualLinkPlaceHolder($id) {

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