Default should_poll to False if assumed_state is True

[balloob]

Description:

Mini PR: if an entity has assumed_state = True, we should default should_poll to False.

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass

[OttoWinter]

Had a quick look through all uses of assumed_state. In all cases, should_poll is set manually overridden anyway. Or, when assumed_state = True, there's no [async_]update method, so no polling needed. 👍

[amelchio]

So would it be a good idea to remove those should_poll while at it?

[balloob]

Let me go and remove all the should_poll properties and make sure it works.

[balloob]

I found too many confusing places that I don't feel comfortable with this change anymore. Let's just keep it as it was. Makes it more predictable.