Remove struct PQArgBlock from server-side header libpq/libpq.h.

This struct is purely a client-side artifact.  Perhaps there was once
reason for the server to know it, but any such reason is lost in the
mists of time.  We certainly don't need two independent declarations
of it.

src/include/libpq/libpq.h

@@ -20,22 +20,6 @@
 #include "lib/stringinfo.h"
 #include "libpq/libpq-be.h"
 
-/* ----------------
- * PQArgBlock
- *		Information (pointer to array of this structure) required
- *		for the PQfn() call.  (This probably ought to go somewhere else...)
- * ----------------
- */
-typedef struct
-{
-	int			len;
-	int			isint;
-	union
-	{
-		int		   *ptr;		/* can't use void (dec compiler barfs)	 */
-		int			integer;
-	}			u;
-} PQArgBlock;
 
 typedef struct
 {