cars-assemble: remove `type-conversion` concept

[ErikSchierboom]

The type-conversion concept should be removed from the cars-assemble exercise. This involves:

• Removing it from the exercise's concepts in the config.json
• Replace the type conversion content from the exercise's introduction.md with a simple sentence how to convert from a float to an int and vice versa.

[jmrunkle]

Already removed the concept from config.json, just need to update the introduction.

NOTE: I am not certain we could technically add type-conversion as a prereq because it may introduce a cycle since cars-assemble teaches a number of basic concepts and sorting-room requires interfaces which is a more advanced concept. We can always revisit that issue later.