fix: remove deprecated switch from isort

The --recursive or --rc flag in isort is now run by default as of isort
5.0 and generates the following warning if still present:

    lib/python3.8/site-packages/isort/main.py:<line>: UserWarning: W0501:
    The following deprecated CLI flags were used and ignored: -rc!

Squash warning.

Makefile

@@ -68,7 +68,7 @@ coverage: clean
 	$(BROWSER) htmlcov/index.html
 
 isort_check: ## check that isort has been run
-	isort --check-only --diff -rc commerce_coordinator/
+	isort --check-only --diff commerce_coordinator/
 
 isort: ## run isort to sort imports in all Python files
 	isort --recursive --atomic commerce_coordinator/