[NET] NEIGHBOUR: Remove unpopular neigh_is_connected().

neigh_is_connected() is not popular at all, and the only user
drivers/net/cxgb3/l2t.c:t3_l2t_update() also have raw (expanded) expression.
Let's expand it and remove the inline function.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>