eal: remove redundant atomic API description

Atomic functions are described in doxygen of the file
lib/librte_eal/common/include/generic/rte_atomic.h
The copies in arch-specific files are redundant
and confuse readers about the genericity of the API.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Shahaf Shuler <shahafs@mellanox.com>