Remove unused variables.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
Rename sum -> _sum.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
Move `use` statements.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
auto merge of #8197 : omasanori/rust/cleanup, r=graydon

Some patches to reduce warnings, as usual.