How can I remove an entry in global configuration with git config?

I'm not sure what you mean by "undo" the change. You can remove the core.excludesfile setting like this:

git config --global --unset core.excludesfile

And of course you can simply edit the config file:

git config --global --edit

...and then remove the setting by hand.

You can use the --unset flag of git config to do this like so:

git config --global --unset user.name
git config --global --unset user.email

If you have more variables for one config you can use:

git config --global --unset-all user.name