Remove duplicate macro definition.

Seems to be a copy-and-pasteo.  Odd that we heard no reports of
compiler warnings about it.

Thomas Munro

src/include/access/xact.h

@@ -225,7 +225,6 @@ typedef struct xl_xact_twophase
 {
 	TransactionId xid;
 } xl_xact_twophase;
-#define MinSizeOfXactInvals offsetof(xl_xact_invals, msgs)
 
 typedef struct xl_xact_origin
 {