rustc: Obsolete the @ syntax entirely

This removes all remnants of `@` pointers from rustc. Additionally, this removes
the `GC` structure from the prelude as it seems odd exporting an experimental
type in the prelude by default.

Closes #14193
[breaking-change]

src/libsyntax/parse/mod.rs

@@ -282,6 +282,7 @@ mod test {
     use std::io;
     use std::io::MemWriter;
     use std::str;
+    use std::gc::GC;
     use codemap::{Span, BytePos, Spanned};
     use owned_slice::OwnedSlice;
     use ast;