Remove ShouldAutoContinue, implemented in only one subclass.
In ThreadPlanStepOverBreakpoint::ShouldAutoContinue, its return value will always be
the inverse of ShouldStop.
In the one call site of ShouldAutoContinue, its value only matters if ShouldStop is
also true. Since that can never be the case (as above, they'll always be opposite), this
function will never apply, and can be removed.