Remove PG_MMAP_FLAGS from mem.h

Remove PG_MMAP_FLAGS from mem.h

Based on name of the macro, it was implied that it could be used for all
mmap() calls on portability grounds.  However, its use is limited to
sysv_shmem.c, for CreateAnonymousSegment().  This commit removes the
declaration, reducing the confusion around it as a portability tweak,
being limited to SysV-style shared memory.

This macro has been introduced in b0fc0df9364d for sysv_shmem.c,
originally.  It has been moved to mem.h in 0ac5e5a7e152 a bit later.

Suggested by: Peter Eisentraut <peter@eisentraut.org>
Author: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Discussion: https://postgr.es/m/CAExHW5vTWABxuM5fbQcFkGuTLwaxuZDEE2vtx2WuMUWk6JnF4g@mail.gmail.com
Discussion: https://postgr.es/m/12add41a-7625-4639-a394-a5563e349322@eisentraut.org