Stop sharing all gallery images on Facebook (2nd PR)

Following on from the previous PR (https://github.com/guardian/frontend/pull/7320) to prevent Facebook sharing from using poor auto-cropped portrait images from galleries, @Jholder112233 commented that this change would have a major fail situation with Pinterest.

The decision has been made by @crifmulholland to sacrifice Pinterest gallery page shares in order to save Facebook sharing for galleries, until we can devise a better way of feeding the image we want used as the default to Facebook.