eventdev: remove experimental label

eventdev: remove experimental label

The eventdev API was introduced in DPDK 17.05 release.
Since then it
- has been reviewed and iterated for 17.08, 17.11 releases
- three drivers were implemented using the API.
- introduced another subsystem like service core and ethdev-eventdev Rx
adapter APIs to abstract the difference between HW and SW
eventdev implementations in a transparent way.
- had extensive use by the app/test-eventdev/ and
examples/eventdev_pipeline_sw_pmd/

I believe the API is now stable and the EXPERIMENTAL label
should be removed.

Signed-off-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
Acked-by: Gage Eads <gage.eads@intel.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>