Fix spawn forever

[ealmloff]

Spawn forever currently removes the future that was spanned when the component is dropped. This changes spawn forever to not add the future to the list of futures that are removed when the component is removed.

Fixes this issue reported on discord:

I made a repo with a minimum reproducible example (https://github.com/lukasturcani/dioxus-future-issue) -- the long and short of it is that if I spawn a future with onclick my future reaches an await point but then never gets completed. 

I tried using both spawn and spawn_forever, as well as just returning the future from the onclick handler -- all lead to the same results. the interesting thing is -- this only happens if my onclick is nested inside a custom component. (the repo has both the working and non working code)