docs: admin-guide: do not mention the 'run a.out user programs' feature

docs: admin-guide: do not mention the 'run a.out user programs' feature

Running a.out user programs with the latest kernel release is a very rare
and uncommon use case nowadays. The support of a.out user programs is only
remaining for the alpha architecture and is not defined and activated in
the architecture's Kconfig (so even the activation of this support requires
to modify the Kconfig file and not just kernel build configuration).

The discussion on a.out support in 2019 (see Link) shows that the support
of a.out user programs is just remaining for a special corner case from
some (alpha architecture) users.

There is no need to point out and mention this special feature to the
general audience of kernel users. Delete the reference to this historic and
special feature.

Link: https://lore.kernel.org/all/CAHk-=wgt7M6yA5BJCJo0nF22WgPJnN8CvViL9CAJmd+S+Civ6w@mail.gmail.com/
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20220720041325.15693-2-lukas.bulwahn@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>