board/qemu: remove the Qemu version from readme.txt

board/qemu: remove the Qemu version from readme.txt

The Qemu version present in readme.txt files was needed when
the Buildroot's Qemu defconfig was tested manually using the
qemu-system-<arch> binary already present on the host.

This information is now incorrect since we are using host-qemu
package, currently at 4.2.0 version, to do a runtime test since
0c7935063851accf3db4c9eb2fc4f11c3da0c391.

For m68k-q800, we can use the upstream qemu since 4.2.0 release
[1].

So, remove this line from the readme.txt.

[1] https://www.qemu.org/2019/12/13/qemu-4-2-0/

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>