Remove ArduinoRuntime hack

[platisd]

Description

Static instantiation of the ArduinoRuntime in Smartcar.h so it can be used as a conditional statement in different class constructors should be removed, as well as the default argument.

It seems that library users are starting off by copying the examples anyway, so it won't be too cumbersome.

Definition of Done

• Smartcar.h does not instantiate an ArduinoRuntime instance, that is left for the user to do.
• Default runtime argument is removed from the constructors that use it.

[platisd]

This will be done after the current iteration of DIT112 finishes to avoid disrupting the student projects.