board/radxa/rock5b: there is no need to specify rootfstype

The ext4 rootfs is mounted as ext4 perfectly fine without rootfstype
specified on the kernel command line.

In order to keep the kernel command line clean, remove the superfluous
kernel parameter.

Signed-off-by: Niklas Cassel <cassel@kernel.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

board/radxa/rock5b/extlinux.conf

@@ -1,4 +1,4 @@
 label Radxa Rock 5b Linux
   kernel /boot/Image
   devicetree /boot/rk3588-rock-5b.dtb
-  append root=/dev/mmcblk1p1 rw rootfstype=ext4 earlycon rootwait
+  append root=/dev/mmcblk1p1 rw earlycon rootwait