Remove offsetof definition

This was only needed to deal with some ancient and no longer supported
systems.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/9a5223a2-3e25-d4fb-f092-01ec17428584%40enterprisedb.com

src/include/c.h

@@ -30,7 +30,7 @@
  *		2)		bool, true, false
  *		3)		standard system types
  *		4)		IsValid macros for system types
- *		5)		offsetof, lengthof, alignment
+ *		5)		lengthof, alignment
  *		6)		assertions
  *		7)		widely useful macros
  *		8)		random stuff
@@ -703,20 +703,9 @@ typedef NameData *Name;
 
 
 /* ----------------------------------------------------------------
- *				Section 5:	offsetof, lengthof, alignment
+ *				Section 5:	lengthof, alignment
  * ----------------------------------------------------------------
  */
-/*
- * offsetof
- *		Offset of a structure/union field within that structure/union.
- *
- *		XXX This is supposed to be part of stddef.h, but isn't on
- *		some systems (like SunOS 4).
- */
-#ifndef offsetof
-#define offsetof(type, field)	((long) &((type *)0)->field)
-#endif							/* offsetof */
-
 /*
  * lengthof
  *		Number of elements in an array.