Hi,
You should better do it via CSS. So actually this way, you won't remove anything, (otherwise the "Custom Field Match" won't work) but you are just hiding the thumbs underneath the product picture.
To hide the thumbs, include this CSS rule:
#FlexibleAdditionalGallery { display:none;}
into this CSS file:
/templates/YOUR_TEMPLATE/html/com_virtuemart/assets/css/flexible.css
To increase the product picture, see this topic (or use search in the forum, there are many topics about it):
www.flexiblewebdesign.com/forum-virtuemart-2-template-fashionmart-support/53-solved-size-of-image-on-product-details-page#190
(the lines that you need to change might little different, but the idea is the same.. cleanMart uses width40 for product picture column, and width60 for right hand side. You can make them width50 & width50, so the picture will be little bigger)
Regards.
Support