btrfs: dev-replace: remove pointless assert in write unlock

The value of blocking_readers is increased only when the lock is taken
for read, no way we can fail the condition with the write lock.

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

fs/btrfs/dev-replace.c

@@ -982,7 +982,6 @@ again:
 
 void btrfs_dev_replace_write_unlock(struct btrfs_dev_replace *dev_replace)
 {
-	ASSERT(atomic_read(&dev_replace->blocking_readers) == 0);
 	write_unlock(&dev_replace->lock);
 }