repo: Drop unused cache variables leftover from pack files

These are leftovers from the packfile code and should have been
deleted in commit: 2a0601efc7

I noticed this now since I wanted to add a new type of caching.

Closes: #795
Approved by: jlebon

src/libostree/ostree-repo-private.h

@@ -92,8 +92,6 @@ struct OstreeRepo {
   OstreeRepoTransactionStats txn_stats;
 
   GMutex cache_lock;
-  GPtrArray *cached_meta_indexes;
-  GPtrArray *cached_content_indexes;
 
   gboolean inited;
   gboolean writable;