Remove EC2 postfix from InvalidInstanceIDMalformedEC2

With new EC2 error code handling based on ec2_code attribute,
there is no justification for the EC2 postfix.

Implements: blueprint ec2-error-codes

Change-Id: I97ede9a3bcba3a6ac48de50fb6f451f8357a4848
Merge "Remove EC2 postfix from InvalidInstanceIDMalformedEC2"