RDMA/mlx5: Remove two unused declarations

Commit e6fb246c ("RDMA/mlx5: Consolidate MR destruction to
mlx5_ib_dereg_mr()") removed mlx5_ib_free_implicit_mr() but left
the declaration.

Commit d98995b4 ("net/mlx5: Reimplement write combining test") left
mlx5_ib_test_wc().

Remove the unused declarations.

Link: https://patch.msgid.link/r/20240816101358.881247-1-yuehaibing@huawei.com


Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>