migration/rdma: Remove qemu_rdma_broken_ipv6_kernel

migration/rdma: Remove qemu_rdma_broken_ipv6_kernel

I hit following error which testing migration in pure RoCE env:
"-incoming rdma:[::]:8089: RDMA ERROR: You only have RoCE / iWARP devices in your
systems and your management software has specified '[::]', but IPv6 over RoCE /
iWARP is not supported in Linux.#012'."

In our setup, we use rdma bind on ipv6 on target host, while connect from source
with ipv4, remove the qemu_rdma_broken_ipv6_kernel, migration just work
fine.

Checking the git history, the function was added since introducing of
rdma migration, which is more than 10 years ago. linux-rdma has
improved support on RoCE/iWARP for ipv6 over past years. There are a few fixes
back in 2016 seems related to the issue, eg:
aeb76df46d11 ("IB/core: Set routable RoCE gid type for ipv4/ipv6 networks")

other fixes back in 2018, eg:
052eac6eeb56 RDMA/cma: Update RoCE multicast routines to use net namespace
8d20a1f0ecd5 RDMA/cma: Fix rdma_cm raw IB path setting for RoCE
9327c7afdce3 RDMA/cma: Provide a function to set RoCE path record L2 parameters
5c181bda77f4 RDMA/cma: Set default GID type as RoCE when resolving RoCE route
3c7f67d1880d IB/cma: Fix default RoCE type setting
be1d325a3358 IB/core: Set RoCEv2 MGID according to spec
63a5f483af0e IB/cma: Set default gid type to RoCEv2

So remove the outdated function and it's usage.

Cc: Peter Xu <peterx@redhat.com>
Cc: Li Zhijian <lizhijian@fujitsu.com>
Cc: Yu Zhang <yu.zhang@ionos.com>
Cc: Fabiano Rosas <farosas@suse.de>
Cc: qemu-devel@nongnu.org
Cc: linux-rdma@vger.kernel.org
Cc: michael@flatgalaxy.com
Signed-off-by: Jack Wang <jinpu.wang@ionos.com>
Tested-by: Li zhijian <lizhijian@fujitsu.com>
Reviewed-by: Michael Galaxy <mrgalaxy@nvidia.com>
Link: https://lore.kernel.org/r/20250402051306.6509-1-jinpu.wang@ionos.com
[peterx: some cosmetic changes]
Signed-off-by: Peter Xu <peterx@redhat.com>