btrfs: remove build fixup for qgroup_account_snapshot

Introduced in 2c1984f2 ("btrfs: build fixup for
qgroup_account_snapshot") as temporary bisectability build fixup.
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/transaction.c

@@ -1311,11 +1311,6 @@ int btrfs_defrag_root(struct btrfs_root *root)
 	return ret;
 }
 
-/* Bisesctability fixup, remove in 4.8 */
-#ifndef btrfs_std_error
-#define btrfs_std_error btrfs_handle_fs_error
-#endif
-
 /*
  * Do all special snapshot related qgroup dirty hack.
  *