Attest: Remove out-dated comment

Remove an out-dated comment about the limitations
that attestation service is not able to sign the token.
This limitation was removed a long ago, comment was left
in the code accidentally.

Change-Id: I3f6e15fa1a62123820970587ecb2dce8d5680c26
Signed-off-by: Tamas Ban <tamas.ban@arm.com>