[webauthn] Remove attestation specification from spec

leshi has just created a new issue for 
https://github.com/w3c/webauthn:

== Remove attestation specification from spec ==
There seems to be growing consensus that detailed specification of 
attestation formats does not belong in this spec.

Perhaps one way is to say that all attestation statements have a 
`type` and a `value`.  Where type is an enum and `value` is a byte 
array.  Each `type` enum will specify an outside reference for how 
servers interpret `value`.  For example, for `type=android-safetynet`,
 `value` should be interpreted as per 
http://developer.android.com/training/safetynet/index.html.  The 
webauthn spec would give this mapping and that's it.

Please view or discuss this issue at 
https://github.com/w3c/webauthn/issues/108 using your GitHub account