app/testpmd: fix flex parser destroy command

app/testpmd: fix flex parser destroy command

Separate flex item destruction function implementation.

Setups with installed JSON development library can use any value in
range [0, FLEX_MAX_PARSERS_NUM - 1] as input flex item ID.
In setups without JSON development library flex item destruction
function is resolved to empty stub.

Fixes: 2d3d84013508 ("app/testpmd: fix flex item flush")
Cc: stable@dpdk.org
Signed-off-by: Gregory Etelson <getelson@nvidia.com>
Reviewed-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>
Reviewed-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>