Updated clean_static make target
Updated clean_static make target

The contents of the static directories are deleted, but the parent directories are retained. This avoids issues with Django being unable to find ecommerce/static/build after running `make clean_static`.

edx/ecommerce,edx/ecommerce,mferenca/HMS-ecommerce,eduNEXT/edunext-ecommerce,edx/ecommerce,mferenca/HMS-ecommerce,mferenca/HMS-ecommerce,edx/ecommerce,eduNEXT/edunext-ecommerce,eduNEXT/edunext-ecommerce,eduNEXT/edunext-ecommerce