Attest: Remove key-id from token header

The unprotected part of the COSE token header, which contains
the key-id in not a mandatory requirement by the PSA initial
attestation specification. This change remove key-id and related
code as part of the code optimization activity.

Change-Id: Ic22fc949d2c8070ad77c5556013d28fe4f955559
Signed-off-by: Tamas Ban <tamas.ban@arm.com>