If I'm correct that you installed ZFS via dkms then it looks like you need to remove it via the same tool.
dkms remove [module/module-version] [-k kernel/arch] [--all]
Removes a module/version or module/version/kernel/arch combination
from the tree. If the module is currently installed, it first
uninstalls it and if applicable, will replace it with its
original_module. Use the --all option in order to remove all
instances for every kernel at once.
Note: You might wish to avoid using the --all option until you are certain that you have obtained the desired results.