scd: Remove unsupported --card-timeout option.

* doc/scdaemon.texi (card-timeout): Remove.
* scd/scdaemon.c (main): Remove oCardTimeout handling.

There was the card-timeout option in GnuPG 2.0, but it was never
implemented correctly. The intention of this option was to allow
sharing smartcard among multiple applications, but this didn't work
well as user's expectation (it only worked with DISCONNECT command).
This is because other parts of scdaemon assumes exclusive access. In
GnuPG 2.1, the support of the option was removed, improving
"DISCONNECT" command always works well without this option.

• GnuPG-bug-id: T3383
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>