How to set delta_metric to identity in pairwise objective

delta_metric have been introduced from version 2 refactoring:
https://github.com/dmlc/xgboost/blob/600be4dbb54ec5cda4cb61a2bb8fc4bce6f3fc58/src/objective/lambdarank_obj.h#L111-L126

respect the way of computing gradient and hessians in 1.7 versions:
https://github.com/dmlc/xgboost/blob/36eb41c960483c8b52b44082663c99e6a0de440a/src/objective/rank_obj.cu#L876-L892

Where is defined delta and which is its default value in master version?

My cases of use don't need any delta function to overweight top elements of each query as I'm optimizing spearman correlation of the whole query.
I would like not use delta function to weight pairs.
Is possible disable it or set to identity function?