device-dax: delete a redundancy check in dev_dax_validate_align()

After we have done the alignment check for the length of each range, the
alignment check for dev_dax_size(dev_dax) is no longer needed, because it
get the sum of the length of each range.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Link: https://lore.kernel.org/r/20201120092057.2144-1-thunder.leizhen@huawei.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>