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/Build.pm

@@ -526,7 +526,7 @@ sub keep : Chained('buildChain') PathPart Args(1) {
         registerRoot $_->path foreach $build->buildoutputs;
     }
 
-    txn_do($c->model('DB')->schema, sub {
+    $c->model('DB')->schema->txn_do(sub {
         $build->update({keep => $keep});
     });