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/el.json

@@ -212,7 +212,6 @@
     "decline": "απόρριψη",
     "Decrypt %(text)s": "Αποκρυπτογράφησε %(text)s",
     "Decryption error": "Σφάλμα αποκρυπτογράφησης",
-    "(default: %(userName)s)": "(προεπιλογή: %(userName)s)",
     "Delete": "Διαγραφή",
     "Default": "Προεπιλογή",
     "Device already verified!": "Η συσκευή έχει ήδη επαληθευτεί!",