REST API: Remove errant annotation from `test_get_items_pagination_headers()` method.

Removes the debugging group notation left in the `WP_Test_REST_Users_Controller` and `WP_Test_REST_Posts_Controller` classes pagination headers method. Introduced in [38832].

Props sanket.parmar.
Fixes #39398.

Built from https://develop.svn.wordpress.org/trunk@39643


git-svn-id: https://core.svn.wordpress.org/trunk@39583 1a063a9b-81f0-0310-95a4-ce76da25c4cd