When should you remove outliers?
If you decided to remove outliers. Please remove them before the split(even not only before a split, it's better to do the entire analysis(stat-testing, visualization) again after removing them, you may find interesting things by doing this).

If you remove outliers in only any one of train/test set it will create more problems.
(EX: An outlier in train set may not be an outlier in combined/full set, also the model will have high variance if you do so)