scd: call update_card_removed only when detecting removal.
* scd/command.c (update_reader_status_file): Add condition ss->status == 0.
To reproduce the bug: (1) insert card,
(2) run "gpg2 --card-status", (3) remove card, (4) invoke "gpg2 --card-edit", (5) invoke some command like "verify"
The last step fails (but with no error message to user).