Remove useless use of bit-masking macros

In this case, the macros SET_8_BYTES(), GET_8_BYTES(), SET_4_BYTES(),
GET_4_BYTES() are no-ops, so we can just remove them.

The plan is to perhaps remove them from the source code altogether, so
we'll start here.

Discussion: https://www.postgresql.org/message-id/5d51721a-69ef-2053-9172-599b539f0628@2ndquadrant.com

src/backend/utils/adt/numeric.c

@@ -354,12 +354,12 @@ typedef struct NumericSumAccum
  */
 #define NUMERIC_ABBREV_BITS (SIZEOF_DATUM * BITS_PER_BYTE)
 #if SIZEOF_DATUM == 8
-#define NumericAbbrevGetDatum(X) ((Datum) SET_8_BYTES(X))
-#define DatumGetNumericAbbrev(X) ((int64) GET_8_BYTES(X))
+#define NumericAbbrevGetDatum(X) ((Datum) (X))
+#define DatumGetNumericAbbrev(X) ((int64) (X))
 #define NUMERIC_ABBREV_NAN		 NumericAbbrevGetDatum(PG_INT64_MIN)
 #else
-#define NumericAbbrevGetDatum(X) ((Datum) SET_4_BYTES(X))
-#define DatumGetNumericAbbrev(X) ((int32) GET_4_BYTES(X))
+#define NumericAbbrevGetDatum(X) ((Datum) (X))
+#define DatumGetNumericAbbrev(X) ((int32) (X))
 #define NUMERIC_ABBREV_NAN		 NumericAbbrevGetDatum(PG_INT32_MIN)
 #endif