md: don't initialize queue limits

Initial queue limits are now set from ->run.  Remove the superfluous
initialization in md_alloc and level_store.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed--by: Song Liu <song@kernel.org>
Tested-by: Song Liu <song@kernel.org>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20240303140150.5435-10-hch@lst.de