Revise CREATE from unpackaged to not use temp tables

[robe]

As discussed in

​https://lists.osgeo.org/pipermail/postgis-users/2020-May/044226.html

People on Amazon RDS evidentally can't upgrade from 2.? to 3.0 because we are using temp tables.

​https://forums.aws.amazon.com/thread.jspa?messageID=938665

They are getting error

SELECT PostGIS_Extensions_Upgrade()
  NOTICE:  Packaging extension postgis_raster
  ERROR:  cannot create temporary table within security-restricted operation
CONTEXT:  SQL statement "CREATE EXTENSION postgis_raster FROM unpackaged"
PL/pgSQL function postgis_extensions_upgrade() line 48 at EXECUTE

I concluded this is most likely caused by our use of temp tables in our unpackaged script.

strk was all for the idea of removing their use as they just store version info that we should be able to stuff in a variable since our upgrade script is in a big DO $$ $$ call.

​https://lists.osgeo.org/pipermail/postgis-devel/2020-May/028548.html

So I'll work on revising and hopefully can get Amazon to try it to see if it fixes their issue.

[robe]

It looks like Amazon dealt with this so I think it's probably not worthwhile fixing as it might break something else given that we have only heard this as an issue on Amazon RDS.