remove leftover pyproject_.toml file (#632)

The intention in !619 was to have only `pyproject.toml` and remove this prior version with the underscore in the name

Authors:
  - Gregory Lee (https://github.com/grlee77)

Approvers:
  - Gigon Bae (https://github.com/gigony)
  - https://github.com/jakirkham

URL: #632