platform: Drop qemu/sifive_u support

With QEMU v4.2 RISC-V changes to improve the emulation fidelity
of 'sifive_u' machine, OpenSBI v0.4 / U-Boot v2019.10 / Linux
kernel v5.3 images built for the SiFive HiFive Unleashed board
can be used out of the box without any special hack. Hence there
is no need for us to continue supporting such a special target in
OpenSBI. Going forward, sifive/fu540 platform can be used on both
real hardware and QEMU 'sifive_u' machine.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>