ARM: dts: at91: remove duplicated entries

Remove duplicated DTC_FLAGS_<board> := -@ entries which intends to enable
the building of device tree overlays.
Commit 724ba675 ("ARM: dts: Move .dts files to vendor sub-directories")
added those entries at the beginning of file w/o removing the already
available entries spread though file.

Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20230721053918.33944-1-claudiu.beznea@tuxon.dev


Signed-off-by: Claudiu Beznea <claudiu.beznea@tuxon.dev>