Remove usage of phf_macros.

It’s a compiler plugin that uses unstable compiler APIs
that are not on a path to stabilization.

With this changes, there is one less thing that might break
when we update the compiler. For example:
https://github.com/sfackler/rust-phf/pull/101

components/script/lib.rs

@@ -23,7 +23,6 @@
 
 #![doc = "The script crate contains all matters DOM."]
 
-#![plugin(phf_macros)]
 #![plugin(plugins)]
 
 extern crate angle;