In Linux-based distributions, you can simply delete all the folders pertaining to Nmap to uninstall Nmap from your machine. If you have installed Nmap from a downloaded source, there will exist an uninstallation script in the same folder that will uninstall Nmap from your machine. Furthermore, if it was installed in the default location, it can be removed using the following commands:

rm -f bin/Nmap bin/nmapfe bin/xnmap
rm -f man/man1/Nmap.1 man/man1/zenmap.1
rm -rf share/Nmap
./bin/uninstall_zenmap