fix(_tests,Makefile): remove integration tests

They are now in https://github.com/deis/workflow-e2e

Fixes deis/workflow-e2e#6

Author: arschles

Makefile

@@ -94,7 +94,5 @@ test-unit:
 test-functional:
 	@echo "Implement functional tests in _tests directory"
 
-test-integration:
-	$(MAKE) -C _tests/ test-setup test-integration
 
 .PHONY: build clean commit-hook full-clean postgres setup-venv test test-style test-unit test-functional