feat(es7): remove _all mapping

The `_all` field was deprecated in Elasticsearch 6 and completely
removed in [Elasticsearch
7](https://www.elastic.co/guide/en/elasticsearch/reference/current/breaking-changes-7.0.html#all-meta-field-removed).

Pelias has disabled this field for quite some time, however now that we
have dropped support for ES5, we can remove this configuration option.
This also moves us towards supporting ES7!

Connects pelias/pelias#831

Author: missinglink

Diff for: mappings/document.js

@@ -186,9 +186,6 @@ var schema = {
   _source: {
     excludes : ['shape','phrase']
   },
-  _all: {
-    enabled: false
-  },
   dynamic: 'strict'
 };
 

Diff for: test/document.js

@@ -185,7 +185,7 @@ module.exports.tests.dynamic_templates = function(test, common) {
 // _all should be disabled
 module.exports.tests.all_disabled = function(test, common) {
   test('_all disabled', function(t) {
-    t.equal(schema._all.enabled, false, '_all disabled');
+    t.false(schema._all, '_all undefined');
     t.end();
   });
 };

Diff for: test/fixtures/expected.json

@@ -1149,9 +1149,6 @@
         "phrase"
       ]
     },
-    "_all": {
-      "enabled": false
-    },
     "dynamic": "strict"
   }
 }