Remove unused UniqueInstance::PtrRange

Summary: `UniqueInstance.cpp` has its own `PtrRange` which it exclusively uses.

Reviewed By: yfeldblum, Mizuchi

Differential Revision: D29685352

fbshipit-source-id: 32658b3ee6fc1830c2c2f27693baefa16026f13e

folly/detail/UniqueInstance.h

@@ -52,10 +52,6 @@ class UniqueInstance {
   struct key_t {};
 
   using Ptr = std::type_info const*;
-  struct PtrRange {
-    Ptr const* b;
-    Ptr const* e;
-  };
   struct Value {
     Ptr tmpl;
     Ptr const* ptrs;