infiniband: drop unknown function from core_priv.h

Delete ibnl_chk_listeners() and its kernel-doc comments from the
core_priv.h header file.  There is no such function.

Fixes: 233c1955 ("RDMA/netlink: Reduce exposure of RDMA netlink functions")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>