btrfs_err_str function is not called from anywhere and is replicated
in the userspace headers for btrfs-progs.

It's removal also fixes the following linux/btrfs.h userspace
compilation error:

/usr/include/linux/btrfs.h: In function 'btrfs_err_str':
/usr/include/linux/btrfs.h:740:11: error: 'NULL' undeclared (first use in this function)
    return NULL;
Suggested-by: NJeff Mahoney <jeffm@suse.com>
Signed-off-by: NDmitry V. Levin <ldv@altlinux.org>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>