You should actually be able to do this without looping through the individual values by utilizing the Remove function in the token editor.

If your target text is always the same (list order doesn't matter) then you can:

1. Use Retrieve Field Values to get ALL of the values
2. Use Assign Field Values
   1. Select the same field
   2. Set it to REPLACE existing values
   3. Set the value from the Retrieve Field Values activity
   4. Right-click and open the Token Editor
   5. Check Apply Function
   6. Click Select Function
   7. Choose "Remove" (not remove at)
   8. Enter the text for your target value

No looping the values or conditional sequences required.

If you want to be safe, you could create a separate multivalue token to hold the edited list, then assign that list to your document field. At the very least, you may want to do that for your initial tests so you can see how the Remove function is working without having to update a document.

A couple of other notes:

1. You need to use Retrieve Field Values because although you can get fields as part of the search, it will not return multivalue tokens for multivalue fields.
2. I highly recommend doing this in batches. When you get above 200 or 300 loops the instance data really starts to accumulate and bog down the workflow's execution time, meaning it might start at 1 second per document, but end up taking 2 mins or more per document by the time you get to document 5,000