Remove unused import std
Remove unused import std

Signed-off-by: Anders Kaseorg <087231934ba8fcf78bc142ce37d193b895ac03c4@mit.edu>

gbersac/rustc-serialize,indefini/rustc-serialize,rust-lang-deprecated/rustc-serialize,DenisKolodin/rustc-serialize,rust-lang/rustc-serialize,ajroetker/rustc-serialize,shepmaster/rustc-serialize,rust-lang-nursery/rustc-serialize