ARM: dts: stm32: Drop linux,default-trigger = "none" from AV96

The default-trigger setting set to none has no impact and triggers DT
dtbs_check warning, remove it:

"
arch/arm/boot/dts/stm32mp157a-dhcor-avenger96.dtb: led: led4:linux,default-trigger: 'oneOf' conditional failed, one must be fixed:
"

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>