Hi Brian,
I had the same question and while I don't have a solution per se, I have a workaround that could help--if your discussions are graded.
Graded discussions are considered "Assignments" by Canvas and so are found on the Assignments page as well as the Discussions page.
You can create a separate Assignment Group and move all the Discussions you don't want into that group. That process is time-consuming in itself, but not as bad as individually deleting the Discussions.
Once you've put them all into a single group, just delete the group. Canvas will prompt you to ask whether you want the assignments in the group deleted or moved to another group. Choose delete and presto, the discussions are gone.
I just did this with a bunch of unneeded discussions in my class and it worked great. I was fortunate in that I'd already put them in their own group, so deleting them was a 1-step process.