Unnecessary checks on Simulation.Timetable

[r.roeterdink]

There are a number of unnecessary checks on Simulation.Timetable in Signals.cs.

These checks are not required because :
 - other checks in the relevant 'if' statement are such that they can either only occur if Simulation.Timetable is true of if Simulation.Timetable is false
 - the method called following the 'if' statement has separate instances for classes Train.cs and TTTrain.cs

[r.roeterdink (r-roeterdink)]

The superflous checks have been removed.
In one situation (method GetSectionStateClearNode), a new instanced method (TrainGetSectionStateClearNode) has been created to remove the need for the 'if' statement.