* mail-source.el (mail-source-read-passwd): Remove. `read-passwd'

* mail-source.el (mail-source-read-passwd): Remove. `read-passwd'
exists in all supported Emacs versions, so we don't need this
compatibility function.
(mail-source-fetch-pop, mail-source-check-pop)
(mail-source-fetch-webmail): Use read-passwd.

* nntp.el (nntp-send-authinfo, nntp-send-nosy-authinfo)
(nntp-open-telnet, nntp-open-via-telnet-and-telnet): Use
read-passwd.

* nnwarchive.el (nnwarchive-open-server): Use read-passwd.

* imap.el (imap-read-passwd): Remove.
(imap-interactive-login): Use read-passwd.

* canlock.el (canlock-read-passwd): Remove.
(canlock-insert-header, canlock-verify): Use read-passwd.

* sieve-manage.el (sieve-manage-read-passwd): Remove.
(sieve-manage-interactive-login): Use read-passwd.

* pop3.el (pop3-read-passwd): Remove.
(pop3-movemail, pop3-get-message-count, pop3-apop): Use
read-passwd.

* pgg.el (pgg-read-passphrase): Simplify.