Unnecessary `toml` dependency

There's a dependency on the `toml` package: https://github.com/DiamondLightSource/httomo-backends/blob/d534edaac27163fb32e9224fcb948406df805b81/pyproject.toml#L52

However, I don't see anywhere in the codebase that this is imported. Also, if I uninstall it from my local dev environment and then:
- run a pipeline with httomo
- or run all httomo-backends tests

I get no error, so I think this dependency isn't needed.