Delete requirements.txt (#1323)

Because we use poetry to manage the dependencies for the saw
python client we do not wish to also have a checked in
requirements.txt. Instead we use the poetry.lock file to be the
one place to keep up-to-date dependency details, and we can
generate requirements.txt on an as-needed basis using the
poetry command:
```
poetry export -f requirements.txt --output requirements.txt
```

Author: Andrew Kent