Remove ch-tests from make sync command

The tests/charm-helpers contents were removed in the last cycle
but the corresponding sync tool was not updated for this charm.

Change-Id: I06fc6263ba31f7e37b45b1bd0af658bcd602254d

Makefile

@@ -18,4 +18,3 @@ bin/charm_helpers_sync.py:
 
 sync: bin/charm_helpers_sync.py
 	$(PYTHON) bin/charm_helpers_sync.py -c charm-helpers-hooks.yaml
-	$(PYTHON) bin/charm_helpers_sync.py -c charm-helpers-tests.yaml