ARM: dts: socfpga: remove extra alias in the ArriaV devkit

commit [2755e187 dts: socfpga: Add DTS entry for adding the stmmac glue
layer for stmmac.] added an extra ethernet alias in the ArriaV devkit
board file. This patch removes it.

Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>