Removing redundant function

At the moment form actions (buttons) have the classes 'action action' as default. This is because the extraClass function adds 'action' and then calls the parent method. The parent then includes the $this->Type() ('action') again.

So I've remove this overloading of extraClass
Merge pull request #1197 from dhensby/patch-2

Remove redundant extraClasses from FieldActions