net: ena: remove set but not used variable 'hash_key'

drivers/net/ethernet/amazon/ena/ena_com.c: In function ena_com_hash_key_allocate:
drivers/net/ethernet/amazon/ena/ena_com.c:1070:50:
 warning: variable hash_key set but not used [-Wunused-but-set-variable]

commit 6a4f7dc8 ("net: ena: rss: do not allocate key when not supported")
introduced this, but not used, so remove it.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>