pkg/linksharing/sharing: remove unused image code

[boshevski]

The previous linksharing placeholder image is not being used as we replaced it with a css only visual. The code was left commented and now we remove that. Previous PR: https://github.com/storj/edge/pull/465

Code Review Checklist (to be filled out by reviewer)

• [x] Are there any authservice's database migrations? Are they forwards and backwards compatible?
• [x] Does the PR describe what changes are being made?
• [x] Does the PR describe why the changes are being made?
• [x] Does the code follow our style guide?
• [x] Does the code follow our testing guide?
• [x] Is the PR appropriately sized? (If it could be broken into smaller PRs, it should be)
• [x] Does the new code have enough tests? (every PR should have tests or justification otherwise. Bug-fix PRs especially)
• [x] Does the new code have enough documentation that answers "how do I use it?" and "what does it do?"? (both source documentation and higher level, diagrams?)
• [x] Does any documentation needs updating?
• [x] Do the database access patterns make sense?
• [ ] Copy the Commit Message Body section contents into the submit prompt upon PR completion