rustdoc: Remove paths from primitive page <title> tags

Currently primitive pages have a title like "std::u8 - Rust" this changes
it to "u8 - Rust" as "std::u8" is the name of a module not a primitive
type.

src/test/rustdoc/prim-title.rs

@@ -0,0 +1,17 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![crate_name = "foo"]
+
+// @has foo/primitive.u8.html '//head/title' 'u8 - Rust'
+// @!has - '//head/title' 'foo'
+#[doc(primitive = "u8")]
+/// `u8` docs
+mod u8 {}