Remove start_upgrade_registration dispatch handler

This dispatch is no longer raised anywhere, so we may as well get rid of the
code that it executes, as well as the state which is only set there.

src/i18n/strings/de_DE.json

@@ -849,7 +849,6 @@
     "Anyone": "Jeder",
     "Are you sure you want to leave the room '%(roomName)s'?": "Bist du sicher, dass du den Raum '%(roomName)s' verlassen willst?",
     "Custom level": "Benutzerdefiniertes Berechtigungslevel",
-    "(default: %(userName)s)": "(Standard: %(userName)s)",
     "Device ID:": "Geräte-ID:",
     "device id: ": "Geräte-ID: ",
     "Device key:": "Geräte-Schlüssel:",
@@ -859,7 +858,6 @@
     "Password:": "Passwort:",
     "Register": "Registrieren",
     "Save": "Speichern",
-    "Setting a user name will create a fresh account": "Die Eingabe eines Benutzernamens wird ein neues Konto erzeugen",
     "Tagged as: ": "Markiert als: ",
     "This Home Server does not support login using email address.": "Dieser Heimserver unterstützt den Login mittels E-Mail-Adresse nicht.",
     "There was a problem logging in.": "Es gab ein Problem beim anmelden.",