fix(Teleport): component with multi roots should be removed when unmounted

[HcySunYang]

Fix: #3156
When removing the Teleport during patching, we cannot use the internal renderer method remove because there may be components in Teleport’s children, so we need to use unmount instead.
BTW: The reason why a component with a single root can be removed is because the vnode.el of the component is the root DOM node of the component, which is not the case for components with multiple roots.

[LinusBorg]

@HcySunYang There's an older PR #2870 which changes the behavior of remove() which you now don't use anymore in favor of unmount().

Does this change solve the issue of #2870 as well? Do you see any conflict?

[HcySunYang]

@LinusBorg This PR did not solve #2870, I think we can merge #2870 first, and then I will deal with the conflict.