qapi: Delete largely misleading "Stability Considerations"

Documentation section "Stability Considerations" dates back to the
early days of QMP (commit 82a56f0d (Monitor: Introduce the
qmp-commands.hx file)).  It became largely misleading years ago.
Delete it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20230425064223.820979-9-armbru@redhat.com>