Only remove delivery variant if it actually exists

It would.  The case I was trying to address here is that elsewhere in satchless, a variant is constructed and assigned to DeliveryGroup, but not actually saved (yet).  In that case, .delete() fails an assert under 1.5.

But you are right that adding this check outside the `try` gives a different failure case.  I'll move the PK check under the try to account for that case.