Remove now obsolete 'audit' command from Makefile

When 'audit' was removed from setup.py, the corresponding shortcut was
left in the Makefile. This commit removes the audit shortcut from
Makefile.

Closes pallets#2773.

Makefile

@@ -1,4 +1,4 @@
-.PHONY: all install-dev test coverage cov test-all tox docs audit release clean-pyc upload-docs ebook
+.PHONY: all install-dev test coverage cov test-all tox docs release clean-pyc upload-docs ebook
 
 all: test
 
@@ -23,9 +23,6 @@ tox: test-all
 docs: clean-pyc install-dev
 	$(MAKE) -C docs html
 
-audit:
-	python setup.py audit
-
 release:
 	python scripts/make-release.py