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 <[email protected]>
Acked-by: Conor Dooley <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

Author: Johan Jonker