Addressing build warnings

Repositories/UsersRepository.cs

@@ -1,6 +1,3 @@
-using System;
-
-using MongoDB.Bson;
 using MongoDB.Driver;
 
 using TextSender_API.Models;
@@ -100,7 +97,7 @@ public class SaltRepository : BaseRepository
 
     public Salt Retrieve(string userId)
     {
-        return this._bookCollection.Find(s => s.UserId.Equals(userId)).FirstOrDefault();
+        return this._bookCollection.Find(s => s.UserId!.Equals(userId)).FirstOrDefault();
     }
 
     public void Update(Salt salt)