Unfortunately, the implementation was not thread safe and was not
sticking to the thread local main context.
In addition to this, it is not entirely clear to me how this API should
behave. The current approach is to simply cancel the transition with the
state machine halting in its current state. Instead, it could also make
sense for cancellation to cause the state machine to return a
G_IO_ERROR_CANCELLED.
As such, simply remove the feature for now. If anyone actually has a
good use-case then we can add it again.
