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.

electron_app/src/electron-main.js

@@ -232,6 +232,10 @@ ipcMain.on('ipcCall', async function(ev, payload) {
             }
             break;
         case 'origin_migrate':
+            // NB. The code to call this has now been removed so this
+            // should never be called. This code itself has been removed
+            // with the migration to the riot-desktop repo, with the
+            // expectation that this will become the new home for this code.
             migratingOrigin = true;
             await migrateFromOldOrigin();
             migratingOrigin = false;