Remove _delete_tokens function from federation controller

There was a TODO to delete tokens based on idps and mappings, but
since this will be done at the middleware level of keystoneclient
(bug 1291157 then there is no include this piece of code.

Change-Id: I0e198c2b7a05e283409aef25e1d7ab93247814fe
Merge "Remove _delete_tokens function from federation controller"