You haven't stated how you installed nmap. You can only use the package manager that you used to install it — to remove it. If you just downloaded the nmap-7.70.dmg, double-clicked that to mount on your Desktop, and then ran the meta package (.mpkg) installer from there, then you cannot use a third-party package manager to uninstall it, nor do the developers provide an uninstaller, or uninstall instructions.
The nmap installer sprays binaries and man pages into the /usr/local/bin, /usr/local/share/man folder locations, in addition to the obvious in /Applications.
On the outside chance that the installation created a package history, run the following in the Terminal.
pkgutil --pkgs | grep -i nmap
If this shows a package from nnmap.org, you can see all of the files that it installed. Omit the angle brackets below, and use the name that you found in the preceding command output:
pkgutil --files <full name of the nmap package from the previous command> | more -r