docs/devel: Remove nested-papr.txt

docs/devel: Remove nested-papr.txt

docs/devel/nested-papr.txt is entirely (apart from the initial
paragraph) a partial copy of the kernel documentation
https://docs.kernel.org/arch/powerpc/kvm-nested.html

There's no benefit to the QEMU docs to converting this to rST,
so instead delete it. Anybody needing to know the API and
protocol for the guest to communicate with the hypervisor
to created nested VMs should refer to the authoratitative
documentation in the kernel docs.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-id: 20240816133318.3603114-1-peter.maydell@linaro.org