Revert "Add rustc::middle::cstrcache for getting c string bufs safely"

This reverts commit 4e136d1.

Author: brson

src/comp/middle/cstrcache.rs

@@ -30,7 +30,6 @@ mod middle {
     mod freevars;
     mod shape;
     mod gc;
-    mod cstrcache;
 
     mod tstate {
         mod ck;