message: Remove unused AstBuilder
diff: @@ -209,7 +209,6 @@ fn plugin_deps() {
  use syntax::source_map::Span;
  use syntax::ast::*;
  use syntax::ext::base::{ExtCtxt, MacEager, MacResult};
  - use syntax::ext::build::AstBuilder;
  #[plugin_registrar]
  pub fn foo(reg: &mut Registry) {
  @@ -306,7 +305,6 @@ fn plugin_to_the_max() {
  use syntax::source_map::Span;
  use syntax::ast::*;
  use syntax::ext::base::{ExtCtxt, MacEager, MacResult};
  - use syntax::ext::build::AstBuilder;
  use syntax::ptr::P;
  #[plugin_registrar]
  