Revert unnecessary GPDB changes to dead code.

In int8.c: No one compiles GPDB with !USE_FLOAT8_BYVAL. I guess it should
work in theory, but it hasn't been tested for ages. This makes int8.c
100% identical to upstream

In ruleutils.c: elog(ERROR) never returns, so this was dead code.

src/backend/utils/adt/int8.c

@@ -689,9 +689,9 @@ int8inc(PG_FUNCTION_ARGS)
 		int64	   *arg = (int64 *) PG_GETARG_POINTER(0);
 		int64		result;
 
-		result = arg + 1;
+		result = *arg + 1;
 		/* Overflow check */
-		if (result < 0 && arg > 0)
+		if (result < 0 && *arg > 0)
 			ereport(ERROR,
 					(errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE),
 					 errmsg("bigint out of range")));

src/backend/utils/adt/ruleutils.c

@@ -5809,10 +5809,7 @@ get_variable(Var *var, int levelsup, bool istoplevel, deparse_context *context)
 
 		tle = get_tle_by_resno(dpns->outer_tlist, var->varattno);
 		if (!tle)
-		{
 			elog(ERROR, "bogus varattno for OUTER_VAR var: %d", var->varattno);
-			return NULL;
-		}
 
 		Assert(netlevelsup == 0);
 		push_child_plan(dpns, dpns->outer_planstate, &save_dpns);