Remove artificial default processors limit

This commit responds to an API change in core migrating from
EsExecutors#boundedNumberOfProcessors to EsExecutors#numberOfProcessors.

Original commit: elastic/x-pack-elasticsearch@87d6fad971

elasticsearch/src/main/java/org/elasticsearch/xpack/watcher/Watcher.java

@@ -169,7 +169,7 @@ public class Watcher implements ActionPlugin, ScriptPlugin {
                     new FixedExecutorBuilder(
                             settings,
                             InternalWatchExecutor.THREAD_POOL_NAME,
-                            5 * EsExecutors.boundedNumberOfProcessors(settings),
+                            5 * EsExecutors.numberOfProcessors(settings),
                             1000,
                             "xpack.watcher.thread_pool");
             return Collections.singletonList(builder);