mvebu: remove support for deprecated DSA bindings

These legacy bindings were removed long time ago from dts, so there's no
need to keep support for them.

Spotted-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Tomasz Maciej Nowak <tomek_n@o2.pl>

target/linux/mvebu/config-4.19

@@ -350,7 +350,6 @@ CONFIG_MV_XOR=y
 CONFIG_NEED_DMA_MAP_STATE=y
 CONFIG_NEON=y
 CONFIG_NET_DSA=y
-CONFIG_NET_DSA_LEGACY=y
 CONFIG_NET_DSA_MV88E6XXX=y
 CONFIG_NET_DSA_MV88E6XXX_GLOBAL2=y
 # CONFIG_NET_DSA_MV88E6XXX_PTP is not set