CID 1518571: Structurally dead code

cache_fill.cc
```
 56  case TS_CACHE_LOOKUP_SKIPPED:
 57    return "TS_CACHE_LOOKUP_SKIPPED";
 58    break;
 59  default:
 60    return "UNKNOWN_CACHE_LOOKUP_EVENT";
 61    break;
 62  }
   	
CID 1518571 (#2 of 2): Structurally dead code (UNREACHABLE)
unreachable: This code cannot be reached: return "UNKNOWN_CACHE_LOOKU....
 63  return "UNKNOWN_CACHE_LOOKUP_EVENT";
 64}
```