style: Remove explicit rust target for bindgen in stylo

This doesn't seem necessary anymore. It was originally added to disable
unions, which is now done with disable_untagged_union.

Differential Revision: https://phabricator.services.mozilla.com/D163662

components/style/build_gecko.rs

@@ -121,12 +121,9 @@ trait BuilderExt {
 
 impl BuilderExt for Builder {
     fn get_initial_builder() -> Builder {
-        use bindgen::RustTarget;
-
         // Disable rust unions, because we replace some types inside of
         // them.
         let mut builder = Builder::default()
-            .rust_target(RustTarget::Stable_1_25)
             .size_t_is_usize(true)
             .disable_untagged_union();