bcache: Kill bch_next_recurse_key()

This dates from before the btree iterator, and now it's finally gone

Signed-off-by: Kent Overstreet <kmo@daterainc.com>

diff --cc drivers/md/bcache/bset.c
Simple merge

diff --cc drivers/md/bcache/bset.h
Simple merge

diff --cc drivers/md/bcache/btree.c
Simple merge