Remove support for the version attribute

First added in #161 this never ended up panning out, so let's remove the
experimental suport which isn't actually used by anything today and hold off on
any other changes until an RFC happens.

crates/shared/src/lib.rs

@@ -24,7 +24,6 @@ pub struct Program {
 #[derive(Deserialize, Serialize)]
 pub struct Import {
     pub module: Option<String>,
-    pub version: Option<String>,
     pub js_namespace: Option<String>,
     pub kind: ImportKind,
 }