dispatcher: remove system.methodHelp support

Since when we wrapped XML-RPC requests in BindingXMLRPC and API the
methodHelp implementation became useless as it's never reached.

Change-Id: I950de5f690c21860417bba0444459e2ca98f9512
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26922
Reviewed-by: Dan Kenigsberg <danken@redhat.com>