How to remove a Go package downloaded with "go get" command?

But is there a simple way to delete the downloaded package (go delete PACKAGE_NAME?) ?

No there is not.

Manually rming is the way to go.

You can just delete it from you disk:

rm -r $GOPATH/src/<PACKAGE_NAME>

This will remove the package completely. Alas, there is no tool or go command for removing packages. But this should be simple enough.