hw/arm/virt: Delete EL3 error checksnow provided in CPU realize

hw/arm/virt: Delete EL3 error checksnow provided in CPU realize

Now that the CPU realize function will fail cleanly if we ask for EL3
when KVM is enabled, we don't need to check for errors explicitly in
the virt board code. The reported message is slightly different;
it is now:
  qemu-system-aarch64: Cannot enable KVM when guest CPU has EL3 enabled
instead of:
  qemu-system-aarch64: mach-virt: KVM does not support Security extensions

We don't delete the MTE check because there the logic is more
complex; deleting the check would work but makes the error message
less helpful, as it would read:
  qemu-system-aarch64: MTE requested, but not supported by the guest CPU
instead of:
  qemu-system-aarch64: mach-virt: KVM does not support providing MTE to the guest CPU

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20210816135842.25302-4-peter.maydell@linaro.org