Remove manifest timestamp

[matthewrsj]

From clearlinux/swupd-client#95:

We've never used this timestamp for anything. Manifests are meant to be immutable once created. So the timestamps should only flow forward and no issues.

In the unfortunate situation of a DevOps group needing to recreate old Manifests, they will have a new timestamp, and a changed hash relative what an old MoM (ie: assume MoM's got recreated, but some clients had downloaded and cached an old MoM, then downloaded a new bundle manifest on bundle add), which leads to an error/warn in the client at runtime.

The error/warn can be addressed via clearlinux/swupd-client#94, but that code path would also have never hit if the manifest did not have the timestamp in it.

Let's simply remove the timestamp. This can be done without a format bump.

[phmccarty]

In #83, I suggested that we consider adding support for SOURCE_DATE_EPOCH instead of removing the timestamp field. Then, a DevOps group can easily export the variable with the correct timestamp before running swupd-server for an older build.