remove unneeded gitignore lines

- coverage-html: coverage>=6.2 writes a .gitignore file
- mypy_cache: mypy>=0.770 writes a .gitignore file
- pytest_cache: pytest>=3.8.1 writes a .gitignore file
- venv: virtualenv>=20.0.21 writes a .gitignore file

Committed via https://github.com/asottile/all-repos