Revert "hygiene: Make sure expansion info is set at most once for a given Mark"

This reverts commit b15785b.

Author: petrochenkov

src/libsyntax_pos/hygiene.rs

@@ -108,14 +108,7 @@ impl Mark {
 
     #[inline]
     pub fn set_expn_info(self, info: ExpnInfo) {
-        HygieneData::with(|data| {
-            let old_info = &mut data.marks[self.0 as usize].expn_info;
-            if let Some(old_info) = old_info {
-                panic!("expansion info is reset for the mark {}\nold: {:#?}\nnew: {:#?}",
-                       self.0, old_info, info);
-            }
-            *old_info = Some(info);
-        })
+        HygieneData::with(|data| data.marks[self.0 as usize].expn_info = Some(info))
     }
 
     #[inline]