Remove mention of emdebian-archive-keyring

The key in the keyring package has had to be
revoked. A new key is in use as a downloadable
file.

Change-Id: I0c634c6d7630e1a22b8775ec0a92a5945998cb8e
(cherry picked from commit 83713ae331adc66254a30fe8469f6be9b5b1dc02)