Render the item-page thumbnail only once

[bram-atmire]

References

Fixes #5821

Description

Removes the duplicate thumbnail on the item page: the advanced attachment cards rendered their own thumbnail in addition to the item thumbnail already shown in the item-page sidebar.

Instructions for Reviewers

This affects the advanced attachment rendering used on the item page when layout.showDownloadLinkAsAttachment: true (the ds-bitstream-attachment cards). Each card rendered a thumbnail (or the "no thumbnail" placeholder) for its bitstream, which duplicated the item-level thumbnail already rendered in the left sidebar (col-md-4), spending prime above-the-fold space on a second copy.

List of changes in this PR:

• Removed the per-attachment thumbnail block from BitstreamAttachmentComponent's template.
• Removed the now-unused thumbnail$ subject, its ngOnInit subscription, the ThemedThumbnailComponent import, and the dead .thumbnail-wrapper style.
• Updated the unit test setup accordingly.

The single item thumbnail rendered in the sidebar is retained; the attachment cards keep their file metadata and download button.

How to test:

1. Set layout.showDownloadLinkAsAttachment: true in your config.*.yml.
2. Open an item that has an attached file: only one thumbnail (the sidebar one) is shown; the attachment card no longer renders its own thumbnail/placeholder.
3. Confirm with a multi-file item and with an item that has no thumbnail (the placeholder also appears only once).

Checklist

• My PR is created against the main branch of code.
• My PR is small in size.
• My PR passes lint validation.
• My PR passes all tests (existing Unit Tests updated).
• My PR includes details on how to test it.
• If my PR includes new libraries/dependencies: n/a.
• If my PR modifies REST API endpoints: n/a.
• If my PR includes new configurations: n/a.
• If my PR fixes an issue ticket, I've linked them together.

[tdonohue]

@bram-atmire : I'm not sure I agree with this PR as it removes the thumbnail entirely from the ds-bitstream-attachment.

My understanding is that this came over from DSpace-CRIS because it's a part of the Configurable Layout of Entity/Item pages coming in DSpace 11, see DSpace/DSpace#11779

I believe the thumbnail appears in this component because it does NOT always appear in the other location, based on how the layout is configured.

So, I think it'd be a better solution to make this configurable... or wait until DSpace/DSpace#11779 is fixed before we move this forward.