Extract getDropColumns into base connector

Author: ssh24

lib/migration.js

@@ -112,13 +112,6 @@ function mixinMigration(PostgreSQL) {
     return sql;
   };
 
-  PostgreSQL.prototype.getDropColumns = function(model, actualFields) {
-    var sql = [];
-    var self = this;
-    sql = sql.concat(self.getColumnsToDrop(model, actualFields));
-    return sql;
-  };
-
   PostgreSQL.prototype.getColumnsToAdd = function(model, actualFields) {
     var self = this;
     var m = self._models[model];