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

Author: asottile

.gitignore

@@ -2,8 +2,5 @@
 *.py[co]
 .*.sw[a-z]
 /.coverage
-/.mypy_cache
-/.pytest_cache
 /.tox
 /dist
-/venv*