delete requirements.tx, rely solely on poetry.lock

[pnwamk]

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

[RyanGlScott]

Yes. YES.