Fix: various problems with minimum order amount enabled

See magento#6151 for the basics.

When minimum order amount is enabled:
1. add 1 product to the cart; total is below min order amount -> no error.
2. Try to remove that product -> error about minimum amount.
3. Try to add more to the cart with total still below min order -> error about minimum amount
4. Add a product that will bring the total cart amount **over** the min order amount and no errors!

I will request the Magento team takes a look at this line. I don't understand why the minimum amount is being validated here but perhaps it has something to do with multi-shipping checkouts. However, it botches the min order amount logic badly in it's current form.

app/code/Magento/Quote/Model/ShippingAddressManagement.php

@@ -117,9 +117,13 @@ public function assign($cartId, \Magento\Quote\Api\Data\AddressInterface $addres
         $address->setSaveInAddressBook($saveInAddressBook);
         $address->setCollectShippingRates(true);
 
+
+        /*
+        // Fix for https://github.com/magento/magento2/issues/6151
         if (!$quote->validateMinimumAmount($quote->getIsMultiShipping())) {
             throw new InputException(__($this->getMinimumAmountErrorMessage()->getMessage()));
         }
+        */
 
         try {
             $address->save();