Deprecate and remove in-memory object caching (#5481)

in-memory caching cannot be cleanly implemented
without the access to GC which Go doesn't naturally
provide. At times we have seen that object caching
is more of an hindrance rather than a boon for
our use cases.

Removing it completely from our implementation
  related to #5160 and #5182

cmd/test-utils_test.go

@@ -1657,12 +1657,6 @@ func initObjectLayer(endpoints EndpointList) (ObjectLayer, []StorageAPI, error)
 		return nil, nil, err
 	}
 
-	// Disabling the cache for integration tests.
-	// Should use the object layer tests for validating cache.
-	if xl, ok := objLayer.(*xlObjects); ok {
-		xl.objCacheEnabled = false
-	}
-
 	// Success.
 	return objLayer, formattedDisks, nil
 }