Remove deprecated language forced field

With elastic#68 we replaced `language`field with `_language`.

We can now remove the old deprecated name.

Closes elastic#69.
(cherry picked from commit e39f144)

Author: dadoonet

src/main/java/org/elasticsearch/index/mapper/attachment/AttachmentMapper.java

@@ -351,10 +351,6 @@ public void parse(ParseContext context) throws IOException {
                         contentType = parser.text();
                     } else if ("_name".equals(currentFieldName)) {
                         name = parser.text();
-                    } else if ("language".equals(currentFieldName)) {
-                        // TODO deprecated form. Will be removed in 2.3
-                    	language = parser.text();
-                        logger.debug("`language` is now deprecated. Use `_language`. See https://github.com/elasticsearch/elasticsearch-mapper-attachments/issues/68");
                     } else if ("_language".equals(currentFieldName)) {
                         language = parser.text();
                     }