Remove unused, unsound method on StrInterner

The string slices returned by `get_ref` can actually be
invalidated by calling `clear`.  Since this method is unused,
it is easiest to simply remove it.

Closes rust-lang#17181

Author: bkoropoff

src/libsyntax/util/interner.rs

@@ -19,7 +19,6 @@ use std::cell::RefCell;
 use std::cmp::Equiv;
 use std::fmt;
 use std::hash::Hash;
-use std::mem;
 use std::rc::Rc;
 
 pub struct Interner<T> {
@@ -192,16 +191,6 @@ impl StrInterner {
         (*self.vect.borrow().get(idx.uint())).clone()
     }
 
-    /// Returns this string with lifetime tied to the interner. Since
-    /// strings may never be removed from the interner, this is safe.
-    pub fn get_ref<'a>(&'a self, idx: Name) -> &'a str {
-        let vect = self.vect.borrow();
-        let s: &str = vect.get(idx.uint()).as_slice();
-        unsafe {
-            mem::transmute(s)
-        }
-    }
-
     pub fn len(&self) -> uint {
         self.vect.borrow().len()
     }