docs/about: Change notes on x86 machine type deprecation into a general one

docs/about: Change notes on x86 machine type deprecation into a general one

We now have a general note about versioned machine types getting
deprecated and removed at the beginning of the deprecated.rst file,
so we should also have a general note about this in removed-features.rst
(which will also apply to versioned non-x86 machine types) instead of
listing individual old machine types in the document.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20250116064644.65670-1-thuth@redhat.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>