front: lmr: remove grid margins

[eckter]

Apparently the grid margin is meant to estimate how long the signals must be green before they're passed by the train. But we already have an accurate estimation with the sight distance, something they don't have in contexts where grid margin is used. So we add two redundant margins and we end up too pessimistic.

We don't want to remove the feature (yet?), but we should set the default value to 0.

See #9984 for the last PR on this subject, we probably need to do the same kind of change again.