Fix migration for CE 3.0.1 to current (#5844)

* Fix migration for CE

* Remove dead code

Author: apata

lib/plausible/data_migration/backfill_tracker_script_configuration.ex

@@ -72,8 +72,16 @@ defmodule Plausible.DataMigration.BackfillTrackerScriptConfiguration do
 
   defp tracker_script_configuration(site, now) do
     installation_meta = site.installation_meta
-    installation_type = if(installation_meta, do: installation_meta.installation_type, else: nil)
-    script_config = if(installation_meta, do: installation_meta.script_config, else: %{})
+
+    installation_type =
+      if installation_meta do
+        Map.get(installation_meta, "installation_type")
+      end
+
+    script_config =
+      if installation_meta do
+        Map.get(installation_meta, "script_config", %{})
+      end
 
     %{
       id: Nanoid.generate(),