remove _timestamp from audit index mapping

we're not using the _timestamp field and the path option is no longer supported
in elasticsearch 2.0 so this commit removes the field from the mapping.

Original commit: elastic/x-pack-elasticsearch@399d835d1f

src/main/resources/shield_audit_log.json

@@ -10,10 +10,6 @@
       "_all" : {
         "enabled" : false
       },
-      "_timestamp" : {
-        "enabled" : true,
-        "path" : "@timestamp"
-      },
       "properties": {
         "@timestamp": {
           "type": "date",