Unload or remove Key if the according Yubikey is not inserted #69

In my case, I usully use multiple Yubikeys (One fixed at my Workplace, one at my Keyring, one as Backup).
But i always only have one of them inserted at a given time.
This leads to the case that the SSH keys for all three Yubikeys are loaded.
Event after a restert of WinCryptSSHAgent the Keys are reloaded because they are present in the Windows Certificate store.

This leads to the following behavior when creating a new SSH connection:

• For ech loaded key windows first asks me if i want to use a Smartcard device
• If this is the device which is currently inserted i can enter the PIN aneferything is fine
• If this is not the device which is currently inserted the action fails and i have to click "abort". After this the next will be tried

A possible better behaviour for this would be to to remove a key as long as the device is not inserted.
This would prevent the errors above.