btrfs: Remove unused function btrfs_account_dev_extents_size

This function is not used since the alloc_start parameter has been
obsoleted in commit 0d0c71b3 ("btrfs: obsolete and remove
mount option alloc_start").

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>