[dovecot/core] e1abe6: auth: Remove userdb nss

[GitHub]

  Branch: refs/heads/master
  Home:   https://github.com/dovecot/core
  Commit: e1abe6d11a9e63995a7e0ff40261973ebc17ad37
      https://github.com/dovecot/core/commit/e1abe6d11a9e63995a7e0ff40261973ebc17ad37
  Author: Timo Sirainen <timo.sirainen at dovecot.fi>
  Date:   2017-11-14 (Tue, 14 Nov 2017)

  Changed paths:
    M src/auth/Makefile.am
    R src/auth/userdb-nss.c
    M src/auth/userdb.c

  Log Message:
  -----------
  auth: Remove userdb nss

Its original purpose was because getpwnam() couldn't differentiate between
"user doesn't exist" and "temporary error", but this was solved by using
getpwnam_r() instead.

So all userdb nss users should be able to safely switch to userdb passwd.