Fix thread pool docs regarding dynamic settings

Thread pool settings are no longer dynamically updatable since
da74323141a69bd06fc19a70bc072143e9f0e986. This commit removes a leftover
note from the thread pool module docs that incorrectly states that
thread pool settings are dynamically updatable.

docs/reference/modules/threadpool.asciidoc

@@ -66,8 +66,6 @@ thread_pool:
         size: 30
 --------------------------------------------------
 
-NOTE: you can update thread pool settings dynamically using <<cluster-update-settings>>.
-
 [float]
 [[types]]
 === Thread pool types