cbq: remove TCA_CBQ_OVL_STRATEGY support

since initial revision of cbq in 2004 iproute 2 has never implemented
support for TCA_CBQ_OVL_STRATEGY, which is what needs to be set to
activate the class->drop() call (TC_CBQ_OVL_DROP strategy must be
set by userspace value must be set by userspace).

David Miller says:
   It seems really safe to kill this thing off, flag an error if someone
   tries to set the attribute, and therefore kill off all of the
   non-default cbq_ovl_*() functions.

A followup commit can then remove all .drop qdisc methods since this
removed the only caller.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>