liveupdate: Remove unused ser field from struct luo_session

liveupdate: Remove unused ser field from struct luo_session

The ser field in struct luo_session was intended to point to the
serialized data for a session, but it was never actually utilized in the
implementation. All serialization and deserialization logic consistently
uses the pointers maintained in struct luo_session_header.

Remove the dead field to clean up the structure.

Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Pratyush Yadav (Google) <pratyush@kernel.org>
Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Link: https://patch.msgid.link/20260527202737.1345192-6-pasha.tatashin@soleen.com
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>