Remove unknown variable: `onclick_attrib`

This has been removed in Django 1.4-1693-gd7b49f5:
https://github.com/django/django/commit/d7b49f5

When using TEMPLATE_STRING_IF_INVALID, this causes an error.
Merge pull request #198 from blueyed/fix-unknown-var-onclick_attrib

DjangoCMS: Remove unknown variable: `onclick_attrib`