I could not see the same menus referred to in the solution above, so here's how I went about it on my Mac:
The topmost row of menus on www.airbnb.com has a "Host" menu. Hover your cursor over it to see the Host menu options. Choose "Calendar." This will open the calendar page for your listing (or one of them, if you have multiple listings). On the left side of the Calendar page will be a column list of options ("Calendar" being the currently selected option); you many need to scroll to get to the "Co-hosts" option. Click it. Now it will show all hosts. Find the co-host to be removed, click the "Remove" link and confirm at the prompt.
If you have multiple listings, you will need to remove the co-host from each one. The second row from the top (a black bar), left-hand side, has a pop-up list of all your listings. Whichever listing you select here is the one you will be removing the co-host from, so select another listing and repeat the process above to remove.
This can be done by the primary host, or the co-host can do it themselves.
