examples/ipsec-secgw: clear soft expiry configuration

examples/ipsec-secgw: clear soft expiry configuration

Soft expiry is not a mandatory IPsec feature. It is verified separately
with IPsec unit tests. So configuration of the same is not required.
Also, soft expiry tracking can cause perf degradation with some PMDs.
Since a separate UT is available and the same setting in ipsec-secgw is
not verifying the functionality, remove the same by clearing life
configuration.

Signed-off-by: Anoob Joseph <anoobj@marvell.com>
Acked-by: Akhil Goyal <gakhil@marvell.com>