REST Tests: Remove human parameter (elastic/x-pack-elasticsearch#4032)

This parameter already exists in the core _common.json, no need to
repeat.

relates elastic/x-pack-elasticsearch#3575

Original commit: elastic/x-pack-elasticsearch@49cd3e0df5

plugin/src/test/resources/rest-api-spec/api/xpack.info.json

@@ -7,10 +7,6 @@
       "paths": [ "/_xpack" ],
       "parts": {},
       "params": {
-        "human" : {
-          "type" : "boolean",
-          "description" : "Presents additional info for humans (feature descriptions and X-Pack tagline)"
-        },
         "categories": {
           "type": "list",
           "description" : "Comma-separated list of info categories. Can be any of: build, license, features"