Remove outdated and possibly unused environment.yaml

[Peque]

The file docs/environment.yaml seems to be outdated as it is not possible to generate the documentation with a virtualenvironment created based on that file. It is currently missing dataclasses (since it declares use of Python 3.6), sphinx-gallery, sphinx-toggleprompt and Pillow.

Since all those dependencies and the dataclasses requirement for Python 3.6 is already handled in setup.py, I think the docs/environment.yaml could be simply removed to avoid confusion and duplicated dependencies declaration.

[wholmgren]

correct, the doc build uses an environment specified by pip install pvlib[doc]