Avoid creating unnecessary travel time calculators

[rakow]

During initialization, a TravelTimeCalculator is created for every network mode defined in the routing. However, not every of these modes are necessarily run on the network as well. E.g. there could be routed only modes like, walk, bike, or ride that have a fixed travel time. These will have a calculator installed that never does anything, but still takes up computing resources because it will process a lot of events.

In this PR the behavior is changed, so that a travel time calculator is only installed for analyzed modes, which can already be defined in the travel time config.