It's very easy to bulk delete collections: first, navigate to Content in the Admin Area, then go to Collections.

Click on the checkbox at the top of your Collection listing.

This will select all checkboxes on that page (if you have more pages, just repeat this process for each page).

Select the Delete option.

You'll be prompted with a confirmation that you truly want to delete all the videos selected, so click Delete Selected Collections, and that's it!