net: sit: Do not set .ndo_get_stats64

If the driver is using the network core allocation mechanism, by setting
NETDEV_PCPU_STAT_TSTATS, as this driver is, then, it doesn't need to set
the dev_get_tstats64() generic .ndo_get_stats64 function pointer. Since
the network core calls it automatically, and .ndo_get_stats64 should
only be set if the driver needs special treatment.

This simplifies the driver, since all the generic statistics is now
handled by core.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>