chore: drop 'dev-requirements.txt' (#23)
chore: drop 'dev-requirements.txt' (#23)

It is not used by any CI or other script.

Motivated by:
https://github.com/googleapis/python-db-dtypes-pandas/pull/21#issuecomment-933659497
googleapis/python-db-dtypes-pandas,googleapis/python-db-dtypes-pandas