fix include_in_all for multi field

As in document http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/mapping-core-types.html#_include_in_all

The `include_in_all` should be ignored for multi-field. Currently it will set the `includeInAll` to `null`, then it will be default to `true` if `index` is not set to `no`. I think it should be set to `false` instead of `null`.

btw, seems `AllFieldMapper.IncludeInAll.unsetIncludeInAll()` will not be used anywhere after this change, should that be removed?

closes #5364