steam usually stores mods in a separate folder from the game folder. go to where steam is installed, and look for a folder:

C:\Program Files (x86)\Steam\SteamApps\workshop

in that folder are all the mods you have ever downloaded from steam. unfortunately, they are all just referenced by number. if it's the last mod you downloaded, you can sort by date. delete whatever the most recent mod there is, AND go into the subfolder "content", which is where the mod files themselves are locate, and go through the folders there (again, all numbered) and find the folder where the mod files you want to delete are located. again, easier if it's the most recent mod you downloaded... otherwise you will have to take your best guess as to which is the proper folder. if in doubt, move it to your trashcan first, run the game, and if it was the right one, great. if not, restore the files from your trashcan and try again.

also? check the folder

C:\Users\[yourname]\Documents\Paradox Interactive\Age of Wonders Planetfall

to see if there is a folder called "Mods" in there.

if there is, you need to manually delete the mod from there as well.