mlxsw: spectrum_router: Remove FIB entry list from FIB node

As explained in previous patches, the driver no longer needs to maintain
a list of identical FIB entries (i.e, same {tb_id, prefix, prefix
length}) and therefore each FIB node can only store one FIB entry.

Remove the FIB entry list and simplify the code.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>