[CLEANUP] - Check the use of MessagePurpose

[shenkeyao]

What is this task and why do we need to work on it?

The MessagePurpose::DaCertificate variant seems unused so we should remove it. We can probably also remove this whole enum and relevant types as well.

What work will need to be done to complete this task?

No response

Are there any other details to include?

No response

What are the acceptance criteria to close this issue?

MessagePurpose related variants and types are removed if unused.

Branch work will be merged to (if not the default branch)

No response