Remove the un-used and non-maintained PAM identity backend

The PAM identity backend has not been maintained nor could it have
functioned since (at least) the grizzly release of OpenStack as the
signatures of the authenticate method (among other methods) do not
match the required signature for the rest of Keystone to utilize it.
With the code bit-rotting and unusable for over two releases it
appears that we could re-evaluate the need for a PAM backend and
implement an auth-plugin that would provide the same basic auth
functionality if there is a demand for it.

Change-Id: I667aa34e252588e7cc840ef765c5f65ebd7d5b62
Merge "Remove the un-used and non-maintained PAM identity backend"