Fix the read barrier config build.

Change-Id: I580d85609e8fa9b8c65466c6596d65bc3b155330
diff --git a/runtime/lambda/closure_builder.cc b/runtime/lambda/closure_builder.cc
index 56bb9fb..9c37db8 100644
--- a/runtime/lambda/closure_builder.cc
+++ b/runtime/lambda/closure_builder.cc
@@ -62,7 +62,6 @@
   if (kIsDebugBuild) {
     if (kUseReadBarrier) {
       UNIMPLEMENTED(FATAL) << "can't yet safely capture objects with read barrier";
-      UNREACHABLE();
     }
   }
 }