app/testpmd: fix flex item flush

app/testpmd: fix flex item flush

Testpmd provides 2 sets of flex item create and destroy functions
One for hosts with JSON library. These functions parse
flex item configuration stored in JSON file and create or destroy
flex item object. The second functions set is for hosts without JSON
library for compilation compatibility.

On hosts without JSON library, current implementation issues
"no JSON library" notification on port close.
The notification was triggered by port destructors that include
flex items flush routine.

The patch introduces single implementation for testpmd
flex item destroy.

Fixes: 59f3a8acbcdb ("app/testpmd: add flex item commands")
Signed-off-by: Gregory Etelson <getelson@nvidia.com>
Reviewed-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>