mbuf: clean old refcnt option

mbuf: clean old refcnt option

CONFIG_RTE_MBUF_SCATTER_GATHER was renamed into CONFIG_RTE_MBUF_REFCNT
by commit 62814bc2e923 and removed by commit 4769bc5a27cc.
Some traces remain because of delayed patches.

It can also be removed from doxygen config.
It is now poisoned in rte_mbuf.h to warn any misuse.

Fixes: d0dff9ba445e ("doc: sample application user guide")
Fixes: fc1f2750a3ec ("doc: programmers guide")
Fixes: 4769bc5a27cc ("mbuf: remove build option to disable refcnt")
Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Changchun Ouyang <changchun.ouyang@intel.com>