RDMA/mlx5: Remove not-used cache disable flag

During execution of mlx5_mkey_cache_cleanup(), there is a guarantee
that MR are not registered and/or destroyed. It means that we don't
need newly introduced cache disable flag.

Fixes: 374012b0 ("RDMA/mlx5: Fix mkey cache possible deadlock on cleanup")
Link: https://lore.kernel.org/r/c7e9c9f98c8ae4a7413d97d9349b29f5b0a23dbe.1695921626.git.leon@kernel.org


Signed-off-by: Leon Romanovsky <leonro@nvidia.com>