nix(loadtest): remove outdated PGRST_DB_URI default

This was required for v9 and earlier, but these don't build with the
current nix invocation anymore anyway. Even loadtesting against v10 does
not work, because `--version` is used in one of the wait scripts and
this was only added in v11.2.

So no need to pretend we'd support comparing against older versions.

nix/tools/loadtest.nix

@@ -53,13 +53,10 @@ let
         workingDir = "/";
       }
       ''
-        # previously required settings to make this work with older branches
         export PGRST_DB_ANON_ROLE="postgrest_test_anonymous"
-        export PGRST_DB_URI="postgresql://"
-        export PGRST_DB_SCHEMAS="test"
-
         export PGRST_DB_CONFIG="false"
         export PGRST_DB_POOL="1"
+        export PGRST_DB_SCHEMAS="test"
         export PGRST_DB_TX_END="rollback-allow-override"
         export PGRST_LOG_LEVEL="crit"
         export PGRST_JWT_SECRET="reallyreallyreallyreallyverysafe"