Update 10/11/2020 to reflect the latest brew changes.
Brew already provide a command to uninstall itself (this will remove everything you installed with Homebrew):
/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/uninstall.sh)"
If you failed to run this command due to permission (like run as second user), run again with sudo
Then you can install again:
/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install.sh)"