try to delete the plugin properly from Admin > Plugins > Local plugins > Manage local plugins. I would expect the uninstall process to delete the web service functions from the database. However if uninstalling the plugin from the admin still cause the error then write a tracker issue, it's a bug.
If you just want to remove the web service functions from a plugin, then you must bump the version.php of your plugin. It will trigger the upgrade process that will update Moodle web service functions (i.e. delete your previously existing local web service functions from Moodle database).