refactor ALTER some-obj SET OWNER implementation

Remove duplicate implementation of catalog munging and miscellaneous
privilege and consistency checks.  Instead rely on already existing data
in objectaddress.c to do the work.

Author: KaiGai Kohei
Tweaked by me
Reviewed by Robert Haas

src/include/nodes/parsenodes.h

@@ -2183,7 +2183,6 @@ typedef struct AlterOwnerStmt
 	RangeVar   *relation;		/* in case it's a table */
 	List	   *object;			/* in case it's some other object */
 	List	   *objarg;			/* argument types, if applicable */
-	char	   *addname;		/* additional name if needed */
 	char	   *newowner;		/* the new owner */
 } AlterOwnerStmt;