removing instantiated game objects after time

Hey guys I’m super new to coding and want to learn.
I am trying to make a simple(not so simple) 2D android game learning as I go.
I am currently stuck where the instantiated gameobjects are not being removed after a certain time period. I don’t have any errors or warnings so I at a loss what to do next although I suspect maybe it’s got somehting to do with this code

IEnumerator removeoldweedfromlist(List list) {
yield return new WaitForSeconds (4.0f);
list.RemoveAll (item => item = null);

I have been trying to figure this out for hours and have realized it must be the item => item part?

Anyway, I have attached the screen shot of the entire script as well.

thank you in advance I applaud anyone who knows anything about coding.

super simple thank you very much