nilfs2: remove nilfs_bmap_delete_block

nilfs_bmap_delete_block() is a wrapper function calling
nilfs_btnode_delete().  This removes it for simplicity.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>

diff --cc fs/nilfs2/bmap.c
Simple merge

diff --cc fs/nilfs2/bmap.h
Simple merge

diff --cc fs/nilfs2/btree.c
Simple merge