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/xl-v1-object_test.go

@@ -184,8 +184,7 @@ func TestGetObjectNoQuorum(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	// Disable caching to avoid returning early and not covering other code-paths
-	xl.objCacheEnabled = false
+
 	// Make 9 disks offline, which leaves less than quorum number of disks
 	// in a 16 disk XL setup. The original disks are 'replaced' with
 	// naughtyDisks that fail after 'f' successful StorageAPI method