Revert mget yaml test changes

Relative to #4892

rest-api-spec/api/mget.json

@@ -32,10 +32,6 @@
           "type" : "boolean",
           "description" : "Refresh the shard containing the document before performing the operation"
         },
-        "source": {
-          "type" : "string",
-          "description" : "The URL-encoded request definition using the Query DSL (instead of using request body)"
-        },
         "_source": {
           "type" : "list",
           "description" : "True or false to return the _source field or not, or a list of fields to return"

rest-api-spec/test/mget/55_source.yaml

@@ -1,27 +0,0 @@
----
-"Source":
-
- - do:
-      indices.create:
-          index: test_1
- - do:
-      cluster.health:
-          wait_for_status: yellow
-
- - do:
-      index:
-          index:   test_1
-          type:    test
-          id:      1
-          body:    { foo: bar }
-
- - do:
-      mget:
-        index:  test_1
-        type:   test
-        source: "{\"ids\":[\"1\"]}"
-
- - is_true:  docs.0.found
- - match:   { docs.0._index:          test_1 }
- - match:   { docs.0._type:           test   }
- - match:   { docs.0._id:             "1"    }