g10: Remove subcommand checkbkupkey for --key-edit.
* g10/keyedit.c (keyedit_menu): Remove cmdCHECKBKUPKEY support.
It was introduced by the commit 9e834047 in 2009. Then, we moved
private key handling to gpg-agent which broke this subcommand.
Note: This subcommand was not supported in 1.4 and 2.0.
- GnuPG-bug-id: T2169