Btrfs: remove non-sense btrfs_error_discard_extent() function

It doesn't do anything special, it just calls btrfs_discard_extent(),
so just remove it.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>
diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c
index edf32c5..d6c03f7 100644
--- a/fs/btrfs/free-space-cache.c
+++ b/fs/btrfs/free-space-cache.c
@@ -2966,8 +2966,8 @@
 	spin_unlock(&block_group->lock);
 	spin_unlock(&space_info->lock);
 
-	ret = btrfs_error_discard_extent(fs_info->extent_root,
-					 start, bytes, &trimmed);
+	ret = btrfs_discard_extent(fs_info->extent_root,
+				   start, bytes, &trimmed);
 	if (!ret)
 		*total_trimmed += trimmed;