scd: Fix duplicate output of KEYPAIRINFO by readkey command.

* scd/app-help.c (app_help_get_keygrip_string_pk): Make HEXKEYGRIP
parm optional.
* scd/command.c (do_readkey): Remove duplicate output of keypairinfo
lines.

Note that this change needs a fix in app-p15 which does not yet handle
the APP_READKEY_FLAG_INFO.

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