Remove GETTHREAD_ALLOWED tracking (#50080)

Remove GETTHREAD_ALLOWED tracking (#50080)

* Remove GETTHREAD_ALLOWED tracking

* removed EE_THREAD_NOT_REQUIRED

* removed EE_THREAD_REQUIRED

* step1

* more changes

* fix Unix build

* more changes

* GetThread in pinvokestubs.S

* build fix for dac

* PInvokeGetThreadHelper

* more changes (dac)

* more (cordbe)

* more (nearly all?)

* Couple places that were mistakengly set to use `GetThreaNotOk` (should use `GetThreadNULLOk`)

* Renamed GetThread() -> GetThreadDoNotUse()

* Missed a few uses of GetThread()

* One more case of GetThread

* Bring GetThread() back - now with an assert in it.

* Renamed PInvokeGetThreadHelper -> JitGetThreadHelper

* One place that should use GetThreadNULLOk (otherwise there are asserts)

* Apply suggestions from code review

Jan's 74 suggestions.

Co-authored-by: Jan Kotas <jkotas@microsoft.com>
* remove SetFiberMode - it is dead code

* Make `EE_THREAD_NOT_REQUIRED` a noop and revert its removal.

* `GetThreadNULLOk()` should be used here.

* PR review suggestion

Co-authored-by: Jan Kotas <jkotas@microsoft.com>
* renamed JitGetThreadHelper -> GetThreadHelper

* One more place that should use `GetThreadNULLOk()`.

Co-authored-by: Jan Kotas <jkotas@microsoft.com>