Reduce repo bloat by removing unused assets

[FloorD]

In 2020 and 2021 particular events were cancelled but assets were already uploaded (think: session info, speaker pictures). Or people ended up cancelling their speaking commitment but their assets weren't removed. Let's make an effort in removing unreferenced assets.

Please submit a PR per event or chapter (n events). Make sure the assets aren't linked somewhere else, using BFG Repo Cleaner, git filter-repo, or similar.

[GeorgeET15]

Hi, can you assign this to me. Thank you

[kritika902]

assign this to me plz