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 #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