Use `findOne` method instead of `findUserBy...`.

Fix eslint error.

src/server/models/external-account.js

@@ -84,15 +84,16 @@ class ExternalAccount {
 
         const User = ExternalAccount.crowi.model('User');
 
-        let promise = User.findUserByUsername(usernameToBeRegistered);
+        let promise = User.findOne({username: usernameToBeRegistered});
         if (isSameUsernameTreatedAsIdenticalUser && isSameEmailTreatedAsIdenticalUser) {
           promise = promise
             .then(user => {
-              if (user == null) { return User.findUserByEmail(mailToBeRegistered) }
+              if (user == null) { return User.findOne({email: mailToBeRegistered}) }
               return user;
             });
-        } else if (isSameEmailTreatedAsIdenticalUser) {
-          promise = User.findUserByEmail(mailToBeRegistered);
+        }
+        else if (isSameEmailTreatedAsIdenticalUser) {
+          promise = User.findOne({email: mailToBeRegistered});
         }
 
         return promise

src/server/models/user.js

@@ -471,13 +471,6 @@ module.exports = function(crowi) {
     return this.findOne({username});
   };
 
-  userSchema.statics.findUserByEmail = function(email) {
-    if (email == null) {
-      return Promise.resolve(null);
-    }
-    return this.findOne({email});
-  };
-
   userSchema.statics.findUserByApiToken = function(apiToken) {
     if (apiToken == null) {
       return Promise.resolve(null);