should I remove unused rgidbodies?

Suppose there are many falling rigidbodies.
When they hit the ground they will stay there till the end.

Is is better to remove the Rigidbody to get static Colliders or is that unnecessary?

It depends on how many Rigidbodies you have and what platform you are planning to use.

If you are developing for mobile you should definitely try to remove all of the unnecessary Rigidbodies. That being said average PC these days can easily support up to hundreds if not thousands of Rigidbodies. But every Rigidbodies does use a little bit of performance even if you don’t notice it.

So long story short you should always try to minimize the amount of Rigidbodies you have in the scene if it’s possible.