Mappings: Allow _version to use disk as a doc values format.

VersionFieldMapper.defaultDocValuesFormat claims that the default is `disk`.
This is not used to choose the DV format in the index but for mappings
serialization in order to know when the _version doc values format is
different from the default format. This made it impossible to use the `disk`
doc values format since mappings would never retain that information at
serialization time.

Close #6523

src/main/java/org/elasticsearch/index/mapper/internal/VersionFieldMapper.java

@@ -102,11 +102,6 @@ public VersionFieldMapper() {
         super(new Names(NAME, NAME, NAME, NAME), Defaults.BOOST, Defaults.FIELD_TYPE, null, null, null, null, docValuesFormat, null, null, null, ImmutableSettings.EMPTY);
     }
 
-    @Override
-    protected String defaultDocValuesFormat() {
-        return "disk";
-    }
-
     @Override
     public void preParse(ParseContext context) throws IOException {
         super.parse(context);