igbvf: remove redundant counter rx_long_byte_count from ethtool statistics

rx_long_byte_count shows the value of the GORC (Good Octets Received
Count) register. However, the register value is already shown as
rx_bytes and they always show the same value.

Remove rx_long_byte_count as the Intel ethernet driver e1000e did in
commit 0a939912 ("e1000e: cleanup redundant statistics counter").

Tested on Intel Corporation I350 Gigabit Network Connection.

Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Kohei Enju <enjuk@amazon.com>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>