Revert r12350 as it didn't fix #2382

Upgrades between 2.1 and 2.2 were fixed by r12641 and refraining
from upgrading a GUC in presence of a pre-installed one is not the
right way to deal with the problem (we'd rather want to replace
an old GUC).

See #2755 (thanks vmo)

git-svn-id: http://svn.osgeo.org/postgis/trunk@12642 b70326c6-7e19-0410-871a-916f4a2858ee