platform: sifive/fu540: Remove "stdout-path" fix-up

As of today the upstream U-Boot & Linux kernel ships a device tree
that already has "stdout-path" properly set in the "/chosen" node.
This is the same with the QEMU 'sifive_u' machine. Hence the codes
to fix up the "stdout-path" in OpenSBI is not necessary.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>