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.

Created by  Maximilian Bosch  on April 10, 2020
CUFVKLLAL54OHDMYUHVJZK46LLX7RJMUJUE5HWYDCGIOQ5TIGXZQC
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