Remove obsolete SAMESIGN macro

The SAMESIGN macro was historically used as a helper for manual
integer overflow checks.  However, since commit 4d6ad3125 introduced
overflow-aware integer operations, this manual sign-checking logic is
no longer necessary.

The macro remains defined in brin_minmax_multi.c and timestamp.c, but
is not used in either file.  This patch removes these definitions to
clean things up.

Author: Richard Guo <guofenglinux@gmail.com>
Discussion: https://postgr.es/m/CAMbWs4-NL3J3hQ3LzrwV-YUkQC18P+jM7ZiegQyAHzgdZev2qg@mail.gmail.com

src/backend/access/brin/brin_minmax_multi.c

@@ -131,8 +131,6 @@ typedef struct MinMaxMultiOptions
 		 ((MinMaxMultiOptions *) (opts))->valuesPerRange : \
 		 MINMAX_MULTI_DEFAULT_VALUES_PER_PAGE)
 
-#define SAMESIGN(a,b) (((a) < 0) == ((b) < 0))
-
 /*
  * The summary of minmax-multi indexes has two representations - Ranges for
  * convenient processing, and SerializedRanges for storage in bytea value.

src/backend/utils/adt/timestamp.c

@@ -48,8 +48,6 @@
 #error -ffast-math is known to break this code
 #endif
 
-#define SAMESIGN(a,b)	(((a) < 0) == ((b) < 0))
-
 /* Set at postmaster start */
 TimestampTz PgStartTime;