bgpd: kill bgp_attr_refcount()

bgpd: kill bgp_attr_refcount()

This attempt at optimization has cost us more than a week's worth of
time on several people hunting down the subtle bug that it was missing
an increment on attr->lcommunity.

This is absolutely not worth the maintenance cost.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>