Refactor users management (#4)

* Improve deletion of 2FA factors

src/data/user.rs

@@ -146,10 +146,6 @@ impl User {
         self.two_factor.push(factor);
     }
 
-    pub fn remove_factor(&mut self, factor_id: FactorID) {
-        self.two_factor.retain(|f| f.id != factor_id);
-    }
-
     pub fn find_factor(&self, factor_id: &FactorID) -> Option<&TwoFactor> {
         self.two_factor.iter().find(|f| f.id.eq(factor_id))
     }