Raster: disable setting the max extent constraint

Disable setting that constraint that seems to consume a lot of resources
when creating tables and is not needed for our use case.

See http://postgis.net/docs/using_raster_dataman.html

This is part of the improvements for #1492
Merge pull request #1508 from CartoDB/fix-big-rasters-import

Raster: disable setting the max extent constraint