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/lib/Hydra/Controller/JobsetEval.pm

@@ -146,7 +146,7 @@ sub release : Chained('evalChain') PathPart('release') Args(0) {
 
     my $release;
 
-    txn_do($c->model('DB')->schema, sub {
+    $c->model('DB')->schema->txn_do(sub {
 
         $release = $c->stash->{project}->releases->create(
             { name => $releaseName