Remove unused method introduced in elastic#113194 (elastic#116793)

(cherry picked from commit 25223dd)

Author: carlosdelest

server/src/main/java/org/elasticsearch/threadpool/ThreadPool.java

@@ -1086,13 +1086,6 @@ public static boolean assertCurrentThreadPool(String... permittedThreadPoolNames
         return true;
     }
 
-    public static boolean assertTestThreadPool() {
-        final var threadName = Thread.currentThread().getName();
-        final var executorName = EsExecutors.executorName(threadName);
-        assert threadName.startsWith("TEST-") || threadName.startsWith("LuceneTestCase") : threadName + " is not a test thread";
-        return true;
-    }
-
     public static boolean assertInSystemContext(ThreadPool threadPool) {
         final var threadName = Thread.currentThread().getName();
         assert threadName.startsWith("TEST-") || threadName.startsWith("LuceneTestCase") || threadPool.getThreadContext().isSystemContext()