Your "login" in this case - the one the system thinks you're trying to remove - is your email.
Since there's only one email, shared between both Google and Facebook, it won't let you remove it.
There's no user-facing functionality to remove just one of the providers attached to that email; it wouldn't even really make sense: since we trust these providers to verify that you own the email address, removing one doesn't actually stop you from logging back into your account with it in the future.
I agree that this a bug, because the UI sets up expectations that it then doesn't fulfill. But the "solution" here is probably a UI change, one that just displays a single email address and then lists the providers you've used with it in the past. We kinda had that at one time though, and it was just as confusing in a bunch of other scenarios - so this is one of those areas where we're probably gonna keep ignoring the situation until some UI wizard figures out a better way of displaying this stuff.
In the meantime, if you really need to remove a provider (let's say you change the email address associated with your Facebook account), email us and, linking to this post, ask us to remove one specific login provider while preserving the email address.