doc: remove flow API migration section

doc: remove flow API migration section

This section has become less relevant since the flow API (rte_flow) is now
a mature DPDK API with applications developed directly on top of it instead
of an afterthought.

This patch removes it for the following reasons:

- It has never been updated to track the latest changes in the legacy
  filter types and never will.

- Many provided examples are theoretical and misleading since PMDs do not
  implement them. Others are obvious.

- Upcoming work on the flow API will alter the behavior of several pattern
  items, actions and in some cases, flow rules, which will in turn cause
  existing examples to be wrong.

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>