qapi: Drop a problematic (Since: 2.11) from query-hotpluggable-cpus

There is a (Since: 2.11) in a query-hotpluggable-cpus example.
Versioning information ought to be in the command description, not
examples.  The command description is basically empty (there is a TODO
about it).

What exactly didn't work before 2.11 is not quite clear from the
documentation.  The example was added in commit 4dc3b15188 (s390x:
implement query-hotpluggable-cpus), which suggests the command failed
for the s390x target until then.  This was almost eight years ago, and
I doubt anyone still cares about this detail.  Simply delete
the problematic (Since: 2.11).

Cc: David Hildenbrand <david@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20250527073916.1243024-6-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

qapi/machine.json

@@ -1089,7 +1089,7 @@
 #    :annotated:
 #
 #    For s390x-virtio-ccw machine type started with
-#    ``-smp 1,maxcpus=2 -cpu qemu`` (Since: 2.11)::
+#    ``-smp 1,maxcpus=2 -cpu qemu``::
 #
 #     -> { "execute": "query-hotpluggable-cpus" }
 #     <- {"return": [