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/Plugin/DarcsInput.pm

@@ -77,7 +77,7 @@ sub fetchInput {
         $sha256 = queryPathHash($storePath);
         $sha256 =~ s/sha256://;
 
-        txn_do($self->{db}, sub {
+        $self->{db}->txn_do(sub {
             $self->{db}->resultset('CachedDarcsInputs')->update_or_create(
                 { uri => $uri
                 , revision => $revision