Remove unnecessary 'use' forms

Fix a laundry list of warnings involving unused imports that glutted
up compilation output.  There are more, but there seems to be some
false positives (where 'remedy' appears to break the build), but this
particular set of fixes seems safe.

src/libsyntax/ext/pipes/ast_builder.rs

@@ -1,4 +1,4 @@
-// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2012-2013 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -19,7 +19,6 @@ use ast::ident;
 use ast;
 use codemap::span;
 
-use core::str;
 use core::vec;
 
 // Transitional reexports so qquote can find the paths it is looking for