Remove nullability compatible tag in scoped_refptr as obsolete

As of 485f2be7c1, this no longer has any effect; instead, the ABSL_NULLABILITY_COMPATIBLE attribute which is already present on the class determines whether a class is compatible with nullability annotations.

Bug: None
Change-Id: I5aeca86c86c2b6eadb2644695ee3621e92f1f568
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/366601
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#43532}

api/scoped_refptr.h

@@ -73,7 +73,6 @@ namespace webrtc {
 template <class T>
 class ABSL_NULLABILITY_COMPATIBLE scoped_refptr {
  public:
-  using absl_nullability_compatible = void;
   using element_type = T;
 
   scoped_refptr() : ptr_(nullptr) {}