Remove the attestation media type field. (#565)

This was never actually correct - these are technically "payloadTypes", which are
not actually mediaTypes. Some implementations mistakenly sent incorrect media types, so
it appeared to work. The GCS storage layer rejected correct implementations that sent the
payloadType, because these are not valid mediaTypes.

We never used this field anyway, so let's drop it. I verified that the API correctly ignores
unknown fields, so removing this will not break clients that send it.

Signed-off-by: Dan Lorenc <lorenc.d@gmail.com>