net: mvneta: Remove unused code

Since the commit 2dcf75e2 ("net: mvneta: Associate RX queues with
each CPU") all the percpu irq are used and disabled at initialization, so
there is no point to disable them first.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>