Revert "Add E040: $(COMMAND) preferred over `COMMAND`"

This reverts commit 43ad17be02017ff6f86e526971fcf8dc4e94adeb.

The world is not quite ready for this (it breaks devstack completely).
We should probably have this as a non-failing warning-level (we
currently don't have the concept of warnings v errors).

Change-Id: Ife310e96fecf9fc297259d6e99ea09a5ef938a33