f2fs: fix to call clear_page_private_reference in .{release,invalid}_folio

b763f3be ("f2fs: restructure f2fs page.private layout") missed
to call clear_page_private_reference() in .{release,invalid}_folio,
fix it, though it's not a big deal since folio_detach_private() was
called to clear all privae info and reference count in the page.

BTW, remove page_private_reference() definition as it never be used.

Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>