The US military presence in Iraq is already far smaller than its presence in Germany and many other countries. Certainly, the US is a global superpower (albeit a declining one) that exerts influence via its military strength. But Iraq is not occupied by the US any more than Germany is.
If Germany wants the US to leave, do they have to negotiate to get that to happen?
It’s hard to say. What would happen if Germany demanded that US forces leave? Germany undoubtedly has the right to do this. I don’t think anyone can say what would happen with any confidence, as it’s quite an outlandish scenario. My guess is that, ultimately, US forces would leave, NATO would collapse, and relations between the US and Germany would decline precipitously. But hey, actions have consequences. This does not mean that Germany cannot get rid of US forces if it wants to; it just means that it doesn’t want to because it would be a bad idea.