refactor(belongs-to-many): remove old code for filtering options.through

This code was used to prevent passing transaction, fields, logging like
options to "create / update" call of junction table. This is no longer
required as we pass junction table data in a different object options.through
now.

lib/associations/belongs-to-many.js

@@ -533,10 +533,7 @@ class BelongsToMany extends Association {
     const updateAssociations = currentRows => {
       const obsoleteAssociations = [];
       const promises = [];
-      let defaultAttributes = options.through || {};
-
-      // Don't try to insert the transaction as an attribute in the through table
-      defaultAttributes = _.omit(defaultAttributes, ['transaction', 'hooks', 'individualHooks', 'ignoreDuplicates', 'validate', 'fields', 'logging']);
+      const defaultAttributes = options.through || {};
 
       const unassociatedObjects = newAssociatedObjects.filter(obj =>
         !_.find(currentRows, currentRow => currentRow[foreignIdentifier] === obj.get(targetKey))
@@ -626,7 +623,7 @@ class BelongsToMany extends Association {
     const targetKey = association.target.primaryKeyAttribute;
     const identifier = association.identifier;
     const foreignIdentifier = association.foreignIdentifier;
-    const defaultAttributes = _.omit(options.through || {}, ['transaction', 'hooks', 'individualHooks', 'ignoreDuplicates', 'validate', 'fields', 'logging']);
+    const defaultAttributes = options.through || {};
 
     newInstances = association.toInstanceArray(newInstances);