net: w5100: remove unused is_w5200()

The is_w5200() function is not used anymore by the commit which adds
the W5500 support.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Mike Sinkovsky <msink@permonline.ru>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>