Remove requirements.txt

[smartspot2]

Heroku recently added support for Poetry (https://devcenter.heroku.com/changelog-items/3050), so we don't actually need a requirements.txt file in the repository anymore.

However, the current pyproject.toml file will not work directly with the buildpack; the Python buildpack on Heroku automatically installs all dependencies in groups, except for those marked with optional = true. Our current setup for pyproject.toml has it the other way around, where production dependencies are marked as optional, while development dependencies are not.

We'd need some way around this, either by flipping the optional status in pyproject.toml, or somehow configuring the buildpack to also install the prod dependency group.