metadata: Remove CrateMetadata::host_lib

It was only used for retreiving edition, which was a bug.
In case of dual proc macros the edition should be taken from the target crate version, like any other metadata.

src/librustc_metadata/creader.rs

@@ -271,7 +271,6 @@ impl<'a> CrateLoader<'a> {
             },
             private_dep,
             span,
-            host_lib,
             raw_proc_macros
         };
 

src/librustc_metadata/cstore.rs

@@ -28,7 +28,6 @@ pub use crate::cstore_impl::{provide, provide_extern};
 pub type CrateNumMap = IndexVec<CrateNum, CrateNum>;
 
 pub use rustc_data_structures::sync::MetadataRef;
-use crate::creader::Library;
 use syntax_pos::Span;
 use proc_macro::bridge::client::ProcMacro;
 
@@ -85,7 +84,6 @@ pub struct CrateMetadata {
     /// for purposes of the 'exported_private_dependencies' lint
     pub private_dep: bool,
 
-    pub host_lib: Option<Library>,
     pub span: Span,
 
     pub raw_proc_macros: Option<&'static [ProcMacro]>,

src/librustc_metadata/decoder.rs

@@ -543,18 +543,13 @@ impl<'a, 'tcx> CrateMetadata {
                 name, SyntaxExtensionKind::Bang(Box::new(BangProcMacro { client })), Vec::new()
             )
         };
-        let edition = if sess.opts.debugging_opts.dual_proc_macros {
-            self.host_lib.as_ref().unwrap().metadata.get_root().edition
-        } else {
-            self.root.edition
-        };
 
         SyntaxExtension::new(
             &sess.parse_sess,
             kind,
             self.get_span(id, sess),
             helper_attrs,
-            edition,
+            self.root.edition,
             Symbol::intern(name),
             &self.get_attributes(&self.entry(id), sess),
         )