Remove Yubikey support

We should remove support for the proprietary Yubikey protocol.

First of all, it is unnecessary, and people should use WebAuthn instead.

Second, otp_yubikey is a culprit in slow startup times (cf. #875 (closed)).

added workflow::discussing label

assigned to @pinguin

created branch 876-remove-yubikey-support to address this issue

mentioned in merge request !1336 (merged)

added part::backend type::bug labels and removed workflow::discussing label

closed with merge request !1336 (merged)

mentioned in commit 60f0e5b5