scd: Change how the removed card flag is set.

* scd/command.c (cmd_serialno): Set/clear card removed flags for all
connections using the current card.

This seems to solve the problems I had in gpg-card and ssh. I am not
sure why the old code was done this way - maybe an oversight when
adding support for multiple cards.

• Signed-off-by: Werner Koch <wk@gnupg.org>