dt-bindings: net: remove arc_emac.txt

The last real user nSIM_700 of the "snps,arc-emac" compatible string in
a driver was removed in 2019. The use of this string in the combined DT of
rk3066a/rk3188 as place holder has also been replaced, so
remove arc_emac.txt

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>