refactor setDBConn to not use step

lib/cartodb/backends/pg_connection.js

@@ -46,36 +46,30 @@ PgConnection.prototype.setDBAuth = function(username, params, callback) {
 // @param callback function(err)
 //
 PgConnection.prototype.setDBConn = function(dbowner, params, callback) {
-    var self = this;
-    // Add default database connection parameters
-    // if none given
     _.defaults(params, {
         dbuser: global.environment.postgres.user,
         dbpassword: global.environment.postgres.password,
         dbhost: global.environment.postgres.host,
         dbport: global.environment.postgres.port
     });
-    step(
-        function getConnectionParams() {
-            self.metadataBackend.getUserDBConnectionParams(dbowner, this);
-        },
-        function extendParams(err, dbParams){
-            assert.ifError(err);
-            // we don't want null values or overwrite a non public user
-            if (params.dbuser !== 'publicuser' || !dbParams.dbuser) {
-                delete dbParams.dbuser;
-            }
-            if ( dbParams ) {
-                _.extend(params, dbParams);
-            }
-            return null;
-        },
-        function finish(err) {
-            callback(err);
-        }
-    );
-};
 
+    this.metadataBackend.getUserDBConnectionParams(dbowner, (err, dbParams) => {
+        if (err) {
+            return callback(err);
+        }
+
+        // we don’t want null values or overwrite a non public user
+        if (params.dbuser !== 'publicuser' || !dbParams.dbuser) {
+            delete dbParams.dbuser;
+        }
+
+        if (dbParams) {
+            _.extend(params, dbParams);
+        }
+
+        callback();
+    });
+};
 
 /**
  * Returns a `cartodb-psql` object for a given username.