Removing Magento sample data, unfortunately, is not as easy as to install it.
Running the command
php bin/magento sampledata:remove
Doesn't really remove all products from your Admin Inventory, from your frontend view, neither from the database. This you'll have to do manually.
What this command does is remove sample data from your composer.json
I think the error Composer throws is associated with the fact that you have already removed the sample-data packages from your composer.json and now running the command again, there is nothing to remove.
Here is what Magento Documentation states about removing sample data:
There is currently no way to uninstall sample data. We recommend you use sample data only to learn about how Magento works. Avoid doing any development in a system in which you installed sample data.
Bummer. Time for some manual labor.