nilfs2: remove obsolete declarations of cache constructor and destructor

The commit 41c88bd7 ("nilfs2: cleanup multi
kmem_cache_{create,destroy} code") consolidated slab constructors and
destructors used in nilfs, but it left some declarations in header
files.

This gets rid of the obsolete declarations.

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