tailq: get rid of broken reserve api

The "reserve" macros and functions do not check if the requested entry is free.
They do nothing more than the lookup function (which itself "creates" entries
...).
The rte_tailq api is marked as "internal use" in documentation and these macros
are only used in test application, so just get rid of them.

Signed-off-by: David Marchand <david.marchand@6wind.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>