Remove the code that calls the origin migrator

This will have done its job now, everyone's had long enough to
install a newer version of Riot and migrate to the new origin.

Laves the code on the backend that handles it for the time being,
as per comment.

src/vector/platform/VectorBasePlatform.js

@@ -173,12 +173,4 @@ export default class VectorBasePlatform extends BasePlatform {
     getDefaultDeviceDisplayName(): string {
         return _t("Unknown device");
     }
-
-    /**
-     * Migrate account data from a previous origin
-     * Used only for the electron app
-     */
-    async migrateFromOldOrigin() {
-        return false;
-    }
 }