Fix hydration race condition

[ryo33]

Fixes #2926

I fixed the issue by removing Interpreter::mount_to_root because its implementation either does nothing or throws the type error, effectively making it dead code.

mount_to_root had passed Node instead of NodeId to appendChildren(id: NodeId, many: number).

[ryo33]

The removed code always fails if the interpreter stack is not empty and does nothing if it is empty, so I suspect the code was added unintendedly.

[jkelleyrtp]

Good catch - rebuild was changed at some point to not require additional appendChildren