net: ena: drop superfluous prototype

Before this commit there was a function prototype named
ena_com_get_ena_admin_polling_mode() that was never implemented.

This patch simply deletes it.

Signed-off-by: Igor Chauskin <igorch@amazon.com>
Signed-off-by: Arthur Kiyanovski <akiyano@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>