Revert "riscv: qemu: clear kernel-start/-end in device tree as workaround for BBL"

The commit was added as a workaround required in QEMU when using BBL as
the supervisor binary interface (SBI) for Linux. We are now using
OpenSBI to provide the SBI, the workaround is therefore not required
anymore and can be removed.

This reverts commit 897206c5.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>