cleanup of unused variables

git-svn-id: http://svn.osgeo.org/postgis/branches/2.0@12174 b70326c6-7e19-0410-871a-916f4a2858ee
Remove use of void pointers in rt_raster_from_gdal_dataset to prevent
void pointer arithmetics. Ticket #2589

git-svn-id: http://svn.osgeo.org/postgis/branches/2.0@12175 b70326c6-7e19-0410-871a-916f4a2858ee