Remove import of now unused AsciiExt

I also replaced a wildcard import with a specific one, while I was
at it.

src/libstd/sys/windows/path.rs

@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use ascii::*;
-
 use path::Prefix;
 use ffi::OsStr;
 use mem;

src/libstd/sys/windows/process.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use ascii::*;
+use ascii::AsciiExt;
 use collections::HashMap;
 use collections;
 use env::split_paths;