Remove now-useless btree_gist--1.2.sql script.

Remove now-useless btree_gist--1.2.sql script.

In the wake of the previous commit, this script will fail
if executed via CREATE EXTENSION, so it's useless.  Remove it,
but keep the delta scripts, allowing old (even very old)
versions of the btree_gist SQL objects to be upgraded to 1.9
via ALTER EXTENSION UPDATE after a pg_upgrade.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Matthias van de Meent <boekewurm+postgres@gmail.com>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/2483812.1754072263@sss.pgh.pa.us