Revert "pre-expansion gate decl_macro"

This reverts commit 1f470ceac2.

src/libsyntax/feature_gate/check.rs

@@ -443,6 +443,11 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                                    "auto traits are experimental and possibly buggy");
             }
 
+            ast::ItemKind::MacroDef(ast::MacroDef { legacy: false, .. }) => {
+                let msg = "`macro` is experimental";
+                gate_feature_post!(&self, decl_macro, i.span, msg);
+            }
+
             ast::ItemKind::OpaqueTy(..) => {
                 gate_feature_post!(
                     &self,

src/test/ui/feature-gates/feature-gate-decl_macro.rs

@@ -2,8 +2,4 @@
 
 macro m() {} //~ ERROR `macro` is experimental
 
-macro_rules! accept_item { ($i:item) => {} }
-accept_item! {
-    macro m() {} //~ ERROR `macro` is experimental
-}
 fn main() {}

src/test/ui/feature-gates/feature-gate-decl_macro.stderr

@@ -7,15 +7,6 @@ LL | macro m() {}
    = note: for more information, see https://github.com/rust-lang/rust/issues/39412
    = help: add `#![feature(decl_macro)]` to the crate attributes to enable
 
-error[E0658]: `macro` is experimental
-  --> $DIR/feature-gate-decl_macro.rs:7:5
-   |
-LL |     macro m() {}
-   |     ^^^^^^^^^^^^
-   |
-   = note: for more information, see https://github.com/rust-lang/rust/issues/39412
-   = help: add `#![feature(decl_macro)]` to the crate attributes to enable
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0658`.