hw/core/resettable: Remove transitional_function machinery

hw/core/resettable: Remove transitional_function machinery

We used to need the transitional_function machinery to handle bus
classes and device classes which still used their legacy reset
handling.  We have now converted all bus classes to three phase
reset, and simplified the device class legacy reset so it is just an
adapting wrapper function around registration of a hold phase method.
There are therefore no more users of the transitional_function
machinery and we can remove it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20240830145812.1967042-12-peter.maydell@linaro.org