btrfs: remove unnecessary fs_info argument from delete_block_group_cache()

The fs_info can be taken from the given block group, so there is no need
to pass it as an argument.

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