DEVDOCS-6162: [UPDATE] remove required variant ID (#701)

<!-- Ticket number or summary of work -->
# [DEVDOCS-6162]


## What changed?
Removed incorrect required label

## Release notes draft

Bug Fix
<!-- Provide an entry for the release notes using simple, conversational
language. Don't be too technical. Explain how the change will benefit
the merchant and link to the feature.

Examples:
* The newly-released [X feature] is now available to use. Now, you’ll be
able to [perform Y action].
* We're happy to announce [X feature], which can help you [perform Y
action].
* [X feature] helps you to create [Y response] using the [Z query
parameter]. Now, you can deliver [ex, localized shopping experiences for
your customers].
* Fixed a bug in the [X endpoint]. Now the [Y field] will appear when
you click [Z option]. -->
* 

## Anything else?
<!-- Add related PRs, salient notes, additional ticket numbers, etc. -->

ping {names}


[DEVDOCS-6162]:
https://bigcommercecloud.atlassian.net/browse/DEVDOCS-6162?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

reference/carts.sf.yml

@@ -1060,7 +1060,6 @@ components:
           required:
             - productId
             - quantity
-            - variantId
           title: Product with a variant    
       x-examples:
         example-1: