auth/KeyRing: always decode keying as plaintext

auth/KeyRing: always decode keying as plaintext

for three reasons:

* we don't encode binary KeyRing since v0.48: the binary encoder for
  KeyRing was dropped in eaea7aa9b28849be612b22ce84971db671319806,
  which was included since v0.48 (argonaut). and we don't encode
  KeyRing in binary manually elsewhere since then.
* we should not use exception in the normal code path. in C++,
  exception is not designed to be efficient or semantically a
  language facility to be part of the normal code path. so, from
  the readability perspective, we should not use exception here.
  as all encoded KeyRings are in plaintext.
* simpler this way.

Signed-off-by: Kefu Chai <kchai@redhat.com>