bcache: Revert "bcache: free heap cache_set->flush_btree in bch_journal_free"

This reverts commit 6268dc2c.

This patch depends on commit c4dc2497 ("bcache: fix high CPU
occupancy during journal") which is reverted in previous patch. So
revert this one too.

Fixes: 6268dc2c ("bcache: free heap cache_set->flush_btree in bch_journal_free")
Signed-off-by: Coly Li <colyli@suse.de>
Cc: stable@vger.kernel.org
Cc: Shenghui Wang <shhuiw@foxmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>