//remove from one list based on like Id's in another list
this.Model.AllContent.RemoveAll(x => this.Model.RelatedContents.Select(y => y.RelatedContentId).Contains(x.Id));
//remove from one list based on like Id's in another list
this.Model.AllContent.RemoveAll(x => this.Model.RelatedContents.Select(y => y.RelatedContentId).Contains(x.Id));