Bluetooth: Mesh: Resolve dead code in store_seg

If CONFIG_BT_MESH_SEQ_STORE_RATE is 1, the check in store_seq can be
reduced to a simple if (false), and the modulo code does not need to be
included in the build.

Fixes #33036.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>