Remove $form_state immutability

Reviving this issue because this code broke a commit :)

I wonder if we should deprecate this feature? My currently feeling is no. We should remove it. Since #2502785: Remove support for $form_state->setCached() for GET requests → it has been a pointless feature and we just didn't get round to completing the work. The 1 usage in contrib I can find is https://git.drupalcode.org/project/sessionless/-/blame/1.x/sessionless_f... - which is a copy and paste of core code and not something we could even issue a deprecation for. The code will not break when we merge this MR. I think all we can do here is issue a CR and open an issue for the sessionless module to remove the unnecessary code.

Change record reads straight forward. Examples are always welcome too.

Changes in MR appear pretty straight forward. All removal and didn't break anything.

Would maybe just target 11.x? And leave alone in 10.x just in case?

@smustgrave yeah this is not for 10.x

Looks good, forgot I opened this one. Committed/pushed to 11.x, thanks!

catch → closed merge request !9340

Automatically closed - issue fixed for 2 weeks with no activity.