Remove long-since-unused file commands/version.h.

Noticed by Itagaki Takahiro.

src/include/commands/version.h

-/*-------------------------------------------------------------------------
- *
- * version.h
- *	  Header file for versions.
- *
- *
- * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
- * Portions Copyright (c) 1994, Regents of the University of California
- *
- * $PostgreSQL: pgsql/src/include/commands/version.h,v 1.21 2009/01/01 17:23:58 momjian Exp $
- *
- *-------------------------------------------------------------------------
- */
-#ifndef VERSION_H
-#define VERSION_H
-
-#endif   /* VERSION_H */