agent: Fix cancellation handling for scdaemon.

* agent/call-scd.c (cancel_inquire): Remove.
(agent_card_pksign, agent_card_pkdecrypt, agent_card_writekey)
(agent_card_scd): Don't call cancel_inquire.

Since libassuan 2.1.0, cancellation command "CAN" is handled within
the library, by assuan_transact. So, cancel_inquire just caused
spurious "CAN" command to scdaemon which resulted an error.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>