[1.2.x] Delete deprecated 'AssetThumbnailViewer' formatter

Problem/Motivation

The purpose of the AssetThumbnailViewer field formatter was to display a media thumbnail by hotlinking the image file directly from Widen. However, this functionality in the 1.1.x branch is taken over by the following combination of configurations:

• A field instance of acquia_dam_asset type (usually the one provided by Drupal\acquia_dam\Entity\MediaSourceField)
  +
  
• Its default acquia_dam_embed_code formatter provided by Drupal\acquia_dam\Plugin\Field\FieldFormatter\EmbedCodeFormatter
  +
  
• Its default remotely_referenced_thumbnail_image universal embed style

The above setup is much more flexible and offers way more possible combinations to fit each Customer's unique requirements, therefore not so much air left around this old AssetThumbnailViewer field formatter.