block: remove BLK_DEV_DAX config option

block: remove BLK_DEV_DAX config option

The functionality for block device DAX was already removed with this
commit:

commit acc93d30d7d4 ("Revert "block: enable dax for raw block devices"")

However, we still had a config option hanging around that was always
disabled because it depended on CONFIG_BROKEN.  This config option was
introduced in this commit:

commit 03cdadb04077 ("block: disable block device DAX by default")

This change reverts that commit, removing the dead config option.

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>