DP: Drop dp_pam_err_to_string
All back end requests were using pam_strerror() to print additional info
about why request failed. Since pam_strerror() returns localized message
and we don't know the locale beforehand, this message failed to be
transferred through D-Bus, resulting in a crash.
Reviewed-by: Sumit Bose <email@example.com>
(cherry picked from commit 8bc6bc6d87127d615f7a81d7151cb46007feff63)