Remove useless pointer update in StatsShmemInit()

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 5891c7a8ed8f.

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