btrfs: remove BUG_ON in btrfs_rm_dev_replace_free_srcdev()

That was only an extra check to tackle a few bugs around this area, now
its safe to remove it.  Replace it by an ASSERT.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>