bcache: remove dupplicated declaration from btree.h

bcache: remove dupplicated declaration from btree.h

Commit 253a99d95d5b ("bcache: move macro btree() and btree_root()
into btree.h") makes two duplicated declaration into btree.h,
	typedef int (btree_map_keys_fn)();
	int bch_btree_map_keys();

The kbuild test robot <lkp@intel.com> detects and reports this
problem and this patch fixes it by removing the duplicated ones.

Fixes: 253a99d95d5b ("bcache: move macro btree() and btree_root() into btree.h")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>