Remove useless LIMIT_OPTION_DEFAULT value from LimitOption

During the development that led to commit 357889eb17, for a time we
had the value LIMIT_OPTION_DEFAULT, which was mostly but not completely
removed later on, before commit.  Complete the removal now.

Author: Zhang Mingli <avamingli@gmail.com>
Discussion: https://postgr.es/m/59d61a1a-3858-475a-964f-24468c97cc67@Spark

src/backend/parser/gram.y

@@ -18461,7 +18461,7 @@ insertSelectOptions(SelectStmt *stmt,
 					 parser_errposition(exprLocation(limitClause->limitCount))));
 		stmt->limitCount = limitClause->limitCount;
 	}
-	if (limitClause && limitClause->limitOption != LIMIT_OPTION_DEFAULT)
+	if (limitClause)
 	{
 		if (stmt->limitOption)
 			ereport(ERROR,

src/include/nodes/nodes.h

@@ -440,7 +440,6 @@ typedef enum LimitOption
 {
 	LIMIT_OPTION_COUNT,			/* FETCH FIRST... ONLY */
 	LIMIT_OPTION_WITH_TIES,		/* FETCH FIRST... WITH TIES */
-	LIMIT_OPTION_DEFAULT,		/* No limit present */
 } LimitOption;
 
 #endif							/* NODES_H */