BackendDB: Get rid of latest-snapshot field

BackendDB: Get rid of latest-snapshot field

All application-level fields should be filled in get_prerelease_data API.
Redis db now contains only data from repos.yaml. if something else is
needed on application level it should be handled in get_prerelease_data
or another application level API.

As latest-snapshot field is not used anywhere it doesn't need to
be moved to get_prerelease_data. So, removing it should be safe.

Change-Id: I442d2be43e8fd1b66a8e6e4e73288b421f0c0f7a
Signed-off-by: Ed Bartosh <eduard.bartosh@intel.com>