Remove make from tox config

[redeboer]

The tox -e doc and related jobs currently work with make. The Makefile can instead be removed, so that the documentation is built with sphinx-build directly. E.g.:

[testenv:doc]
description =
  Build documentation with Sphinx
allowlist_externals =
  sphinx-build
changedir = docs
passenv =
  READTHEDOCS_VERSION
  TERM
commands =
  sphinx-build -nW --keep-going -b html ./ ./_build/html

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