net: ena: Remove an unused field

Remove io_sq->header_addr field because it is no longer
in use.
LLQ was updated to support a bounce buffer so there is
no need in saving the header address of the sq.

Signed-off-by: Nati Koler <nkoler@amazon.com>
Signed-off-by: David Arinzon <darinzon@amazon.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>