Enforce at most one type. (#24428)

This is a follow-up to #24317, which did the hard work but was merged in such a
way that it exposes the setting while still allowing indices to have multiple
types by default in order to give time to people who test against master to
add this setting to their index settings.

core/src/main/java/org/elasticsearch/index/mapper/MapperService.java

@@ -99,12 +99,10 @@ public enum MergeReason {
     public static final Setting<Boolean> INDEX_MAPPING_SINGLE_TYPE_SETTING;
     static {
         Function<Settings, String> defValue = settings -> {
-            // TODO: uncomment this
-            /*boolean singleType = true;
+            boolean singleType = true;
             if (settings.getAsVersion(IndexMetaData.SETTING_VERSION_CREATED, null) != null) {
                 singleType = Version.indexCreated(settings).onOrAfter(Version.V_6_0_0_alpha1_UNRELEASED);
-            }*/
-            boolean singleType = false;
+            }
             return Boolean.valueOf(singleType).toString();
         };
         INDEX_MAPPING_SINGLE_TYPE_SETTING = Setting.boolSetting("index.mapping.single_type", defValue, Property.IndexScope, Property.Final);

core/src/test/java/org/elasticsearch/index/mapper/MapperServiceTests.java

@@ -310,7 +310,6 @@ public void testIndexSortWithNestedFields() throws IOException {
             containsString("cannot have nested fields when index sort is activated"));
     }
 
-    @AwaitsFix(bugUrl="https://github.com/elastic/elasticsearch/pull/24317#issuecomment-297624290")
     public void testForbidMultipleTypes() throws IOException {
         String mapping = XContentFactory.jsonBuilder().startObject().startObject("type").endObject().endObject().string();
         MapperService mapperService = createIndex("test").mapperService();