Tests: Fixing xpack info tests

Even though HEAD is a possible method, this implies that no data is returned
and thus the tests fail randomly.

If HEAD should be added to the api it needs it's own API definition IMO.

Original commit: elastic/x-pack-elasticsearch@a216393f6b

elasticsearch/x-pack/src/test/resources/rest-api-spec/api/xpack.info.json

@@ -1,7 +1,7 @@
 {
   "xpack.info": {
     "documentation": "Retrieve information about xpack, including buid number/timestamp and license status",
-    "methods": [ "GET", "HEAD" ],
+    "methods": [ "GET" ],
     "url": {
       "path": "/_xpack",
       "paths": [ "/_xpack" ],