removed the activation of postgis extension as this is already done i…

…n the testdata.sql script

ci/travis/linux/before_script.sh

@@ -1,5 +1,4 @@
 printf "[qgis_test]\nhost=localhost\ndbname=qgis_test\nuser=postgres" > ~/.pg_service.conf
 psql -c 'CREATE DATABASE qgis_test;' -U postgres
-psql -c 'CREATE EXTENSION postgis;' -U postgres -d qgis_test
 psql -f $TRAVIS_BUILD_DIR/tests/testdata/provider/testdata.sql -U postgres -d qgis_test