Remove unused imports.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
Prefix _ to unused variables.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
Append ; to #[allow(dead_code)].

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
auto merge of #11918 : omasanori/rust/reduce-warnings, r=alexcrichton

Moving forward to green waterfall.