Remove useless AsciiExt imports.

These warn with the next Nightly.

components/selectors/attr.rs

@@ -4,7 +4,6 @@
 
 use cssparser::ToCss;
 use parser::SelectorImpl;
-#[allow(unused_imports)] use std::ascii::AsciiExt;
 use std::fmt;
 
 #[derive(Clone, Eq, PartialEq)]

components/selectors/parser.rs

@@ -14,7 +14,6 @@ use precomputed_hash::PrecomputedHash;
 use servo_arc::ThinArc;
 use sink::Push;
 use smallvec::SmallVec;
-#[allow(unused_imports)] use std::ascii::AsciiExt;
 use std::borrow::{Borrow, Cow};
 use std::fmt::{self, Display, Debug, Write};
 use std::iter::Rev;

components/style_traits/viewport.rs

@@ -7,7 +7,6 @@
 use {CSSPixel, CssWriter, ParseError, PinchZoomFactor, ToCss};
 use cssparser::Parser;
 use euclid::TypedSize2D;
-#[allow(unused_imports)] use std::ascii::AsciiExt;
 use std::fmt::{self, Write};
 
 define_css_keyword_enum! {