hw/arm/vexpress-a9: Remove useless mapping of RAM at address 0

On the vexpress-a9 board we try to map both RAM and flash to address 0,
as seen in "info mtree":

address-space: memory
  0000000000000000-ffffffffffffffff (prio 0, i/o): system
    0000000000000000-0000000003ffffff (prio 0, romd): alias vexpress.flashalias @vexpress.flash0 0000000000000000-0000000003ffffff
    0000000000000000-0000000003ffffff (prio 0, ram): alias vexpress.lowmem @vexpress.highmem 0000000000000000-0000000003ffffff
    0000000010000000-0000000010000fff (prio 0, i/o): arm-sysctl
    0000000010004000-0000000010004fff (prio 0, i/o): pl041
(etc)

The flash "wins" and the RAM mapping is useless (but also harmless).

This happened as a result of commit 6ec1588e in 2014, which changed
"we always map the RAM to the low addresses for vexpress-a9" to "we
always map flash in the low addresses", but forgot to stop mapping
the RAM.

In real hardware, this low part of memory is remappable, both at
runtime by the guest writing to a control register, and configurably
as to what you get out of reset -- you can have the first flash
device, or the second, or the DDR2 RAM, or the external AXI bus
(which for QEMU means "nothing there").  In an ideal world we would
support that remapping both at runtime and via a machine property to
select the out-of-reset behaviour.

Pending anybody caring enough to implement the full remapping
behaviour:
 * remove the useless mapped-but-inaccessible lowram MR
 * document that QEMU doesn't support remapping of low memory

Fixes: 6ec1588e ("hw/arm/vexpress: Alias NOR flash at 0 for vexpress-a9")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1761
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20231103185602.875849-1-peter.maydell@linaro.org

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