To uninstall Custom Properties all elements it constists of have to be uninstalled separately: Extensions > Install / Uninstall
- plg_cptags (content plugin)
- plg_cptags (search plugin)
- plg_cptags (editors-xtd)
When you uninstall the component, all files and database tables get deleted too.
It means that all your works with this component (fields, values, joins, translated values) will be lost. You have been warned: this is not the way to perform upgrades. See relevant document for a detailed procedure.