Remove Hydra::Helper::nix::txn_do from the Perl code

To quote the function's comment:

  Awful hack to handle timeouts in SQLite: just retry the transaction.
  DBD::SQLite *has* a 30 second retry window, but apparently it
  doesn't work.

Since SQLite is now dropped entirely, this wrapper can be removed
completely.

src/script/hydra-create-user

@@ -54,7 +54,7 @@ die "$0: type must be `hydra' or `google'\n"
 
 my $db = Hydra::Model::DB->new();
 
-txn_do($db, sub {
+$db->txn_do(sub {
     my $user = $db->resultset('Users')->find({ username => $renameFrom // $userName });
     if ($renameFrom) {
         die "$0: user `$renameFrom' does not exist\n" unless $user;