configs/raspberrypi{3, 4}_64: enabling BR2_LINUX_KERNEL_DTB_OVERLAY_SUPPORT no longer needed

BR2_LINUX_KERNEL_DTB_OVERLAY_SUPPORT is now forcefully selected by
BR2_PACKAGE_RPI_FIRMWARE_INSTALL_DTB_OVERLAYS when the kernel is in
charge of building DTBs (BR2_LINUX_KERNEL_DTS_SUPPORT=y). So enabling
BR2_LINUX_KERNEL_DTB_OVERLAY_SUPPORT is no longer needed in the 64-bit
defconfigs for Raspberry Pi 3 and 4.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>