add testcase & remove unused code

src/main/java/com/alibaba/druid/sql/parser/Keywords.java

@@ -116,6 +116,8 @@ public class Keywords {
         map.put("USE", Token.USE);
         
         map.put("REPLACE", Token.REPLACE);
+        
+        map.put("COMMENT", Token.COMMENT);
 
         DEFAULT_KEYWORDS = new Keywords(map);
     }

src/main/java/com/alibaba/druid/sql/parser/SQLStatementParser.java

@@ -228,6 +228,11 @@ public class SQLStatementParser extends SQLParser {
                 lexer.nextToken();
                 continue;
             }
+            
+            if (lexer.token() == Token.COMMENT) {
+                statementList.add(this.parseComment());
+                continue;
+            }
 
             throw new ParserException("syntax error, " + lexer.token() + " " + lexer.stringVal() + ", pos "
                                       + lexer.pos());