net: ena: removed unused tx_bytes variable

net: ena: removed unused tx_bytes variable

clang 16.0.0 with W=1 reports:

drivers/net/ethernet/amazon/ena/ena_netdev.c:1901:6: error: variable 'tx_bytes' set but not used [-Werror,-Wunused-but-set-variable]
        u32 tx_bytes = 0;

The variable is not used so remove it.

Signed-off-by: Simon Horman <horms@kernel.org>
Acked-by: Shay Agroskin <shayagr@amazon.com>
Link: https://lore.kernel.org/r/20230328151958.410687-1-horms@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>