btrfs: remove the refcount warning/check at btrfs_put_delayed_ref()

At btrfs_put_delayed_ref(), it's pointless to have a WARN_ON() to check if
the refcount of the delayed ref is zero. Such check is already done by the
refcount_t module and refcount_dec_and_test(), which loudly complains if
we try to decrement a reference count that is currently 0.

The WARN_ON() dates back to the time when used a regular atomic_t type
for the reference counter, before we switched to the refcount_t type.
The main goal of the refcount_t type/module is precisely to catch such
types of bugs and loudly complain if they happen.

This also reduces a bit the module's text size.
Before this change:

   $ size fs/btrfs/btrfs.ko
      text	   data	    bss	    dec	    hex	filename
   1612483	 167145	  16864	1796492	 1b698c	fs/btrfs/btrfs.ko

After this change:

   $ size fs/btrfs/btrfs.ko
      text	   data	    bss	    dec	    hex	filename
   1612371	 167073	  16864	1796308	 1b68d4	fs/btrfs/btrfs.ko

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>