Use ES build instead of xpack inside xpack info action (#122859) (#122869)

Xpack used to be a plugin to Elasticsearch, and existed in a separate
repository. So it had a separate build hash and date, which is exposed
through the xpack info API. Since xpack is no longer a plugin, and part
of the Elasticsearch repository, having a separate build info is no
longer necessary. This commit keeps the xpack info content intact, but
switches the implemntation to look at the Elasticsearch build details,
and removes the xpack build info class.

Author: rjernst

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/XPackBuild.java

@@ -6,6 +6,7 @@
  */
 package org.elasticsearch.xpack.core.action;
 
+import org.elasticsearch.Build;
 import org.elasticsearch.action.ActionListener;
 import org.elasticsearch.action.ActionType;
 import org.elasticsearch.action.support.ActionFilters;
@@ -23,7 +24,6 @@
 import org.elasticsearch.protocol.xpack.XPackInfoResponse.LicenseInfo;
 import org.elasticsearch.tasks.Task;
 import org.elasticsearch.transport.TransportService;
-import org.elasticsearch.xpack.core.XPackBuild;
 
 import java.util.HashSet;
 import java.util.List;
@@ -58,7 +58,7 @@ protected void doExecute(Task task, XPackInfoRequest request, ActionListener<XPa
 
         XPackInfoResponse.BuildInfo buildInfo = null;
         if (request.getCategories().contains(XPackInfoRequest.Category.BUILD)) {
-            buildInfo = new XPackInfoResponse.BuildInfo(XPackBuild.CURRENT.shortHash(), XPackBuild.CURRENT.date());
+            buildInfo = new XPackInfoResponse.BuildInfo(Build.current().hash(), Build.current().date());
         }
 
         LicenseInfo licenseInfo = null;