riscv: qemu: Remove out-of-date "riscv, kernel-start" handling

Commit 66ffe578 ("riscv: qemu: detect and boot the kernel passed by QEMU")
added some logic to handle "riscv,kernel-start" in DT and stored the
address to an environment variable kernel_start.

However this "riscv,kernel-start" has never been an upstream DT binding.
The upstream QEMU never generates such a DT either. Presumably U-Boot
development was based on a downstream QEMU fork.

Now we drop all codes in commit 66ffe578, except that BOARD_LATE_INIT
is kept for later use.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Rick Chen <rick@andestech.com>
Reviewed-by: Simon Glass <sjg@chromium.org>