app/testpmd: fix flow transfer proxy port handling

app/testpmd: fix flow transfer proxy port handling

The current approach detects the proxy port on each port (re-)plug and
may spam the log with error messages if the PMD does not support flows.
As testpmd is a debug tool, it must not do such implicit port handling.
Instead, the new API should be called only when the user requests that.

Revoke the existing code. Implement an explicit command-line primitive
to let the user find the proxy port themselves. Provide relevant hints.

Fixes: 1179f05cc9a0 ("ethdev: query proxy port to manage transfer flows")

Signed-off-by: Ivan Malov <ivan.malov@oktetlabs.ru>
Reviewed-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Acked-by: Ori Kam <orika@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>