wireguard: selftests: cleanup CONFIG_ENABLE_WARN_DEPRECATED

CONFIG_ENABLE_WARN_DEPRECATED is gone since commit 771c0353
("deprecate the '__deprecated' attribute warnings entirely and for
good").

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: David S. Miller <davem@davemloft.net>