MAINT: Remove references to tox in Makefile

Makefile

@@ -8,7 +8,6 @@ help:
 	@echo "clean-test - remove test and coverage artifacts"
 	@echo "lint - check style with flake8"
 	@echo "test - run tests quickly with the default Python"
-	@echo "test-all - run tests on every Python version with tox"
 	@echo "coverage - check code coverage quickly with the default Python"
 	@echo "docs - generate Sphinx HTML documentation, including API docs"
 	@echo "release - package and upload a release"
@@ -32,7 +31,6 @@ clean-pyc:
 	find . -name '__pycache__' -exec rm -fr {} +
 
 clean-test:
-	rm -fr .tox/
 	rm -f .coverage
 	rm -f coverage.xml
 	rm -fr htmlcov/
@@ -48,9 +46,6 @@ test: clean-test
 visualize-tests:
 	python tools/visualize_tests.py
 
-test-all:
-	tox
-
 coverage:
 	coverage report -m
 	coverage html