ethdev: declare Tx prepare API as not experimental

ethdev: declare Tx prepare API as not experimental

The Tx prepare API was introduced at a time when the experimental API
tag mechanism did not exist yet and is missing the tag.
However, this API has been there since 17.02 and did not suffer any
change since its introduction.

Let's just remove the warning from the documentation.

Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>