Remove useless pointer update in StatsShmemInit()

This pointer was not used after its last update.  This variable
assignment was most likely a vestige artifact of the earlier versions of
the patch set that have led to 5891c7a8ed.

This pointer update is useless, so let's remove it.  It removes one call
to pgstat_dsa_init_size(), making the code slightly easier to grasp.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/aKLsu2sdpnyeuSSc@ip-10-97-1-34.eu-west-3.compute.internal

src/backend/utils/activity/pgstat_shmem.c

@@ -180,7 +180,6 @@ StatsShmemInit(void)
 		 * provides a small efficiency win.
 		 */
 		ctl->raw_dsa_area = p;
-		p += MAXALIGN(pgstat_dsa_init_size());
 		dsa = dsa_create_in_place(ctl->raw_dsa_area,
 								  pgstat_dsa_init_size(),
 								  LWTRANCHE_PGSTATS_DSA, NULL);