Eclipse on Mac doesn't allow to downgrade (E)Git
Like it says in the dialog, the Git Integration is required by something else you have installed. There's little unique about the "Eclipse for Committers" download, meaning if you can't uninstall something you want to uninstall, you can still start from the bare basics and then install more into it. You should start with an SDK download from https://download.eclipse.org/eclipse/downloads/ and then add the versions of software you actually want to it.