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 4dc3b151882 (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>