Fix default minimum value for descending sequences

For some reason that is lost in history, a descending sequence would
default its minimum value to -2^63+1 (-PG_INT64_MAX) instead of
-2^63 (PG_INT64_MIN), even though explicitly specifying a minimum value
of -2^63 would work.  Fix this inconsistency by using the full range by
default.

Reported-by: Daniel Verite <daniel@manitou-mail.org>
Reviewed-by: Michael Paquier <michael.paquier@gmail.com>

src/include/pg_config_manual.h

@@ -50,12 +50,6 @@
  */
 #define PARTITION_MAX_KEYS	32
 
-/*
- * Set the upper and lower bounds of sequence values.
- */
-#define SEQ_MAXVALUE	PG_INT64_MAX
-#define SEQ_MINVALUE	(-SEQ_MAXVALUE)
-
 /*
  * When we don't have native spinlocks, we use semaphores to simulate them.
  * Decreasing this value reduces consumption of OS resources; increasing it