[profiler] Remove declaration of unused SamplingAllocationObserver

Change-Id: Ia071e8cfcb021ff7e80888370cafcc58c6cb0fca
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2317947
Auto-Submit: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69070}

src/profiler/sampling-heap-profiler.h

@@ -22,8 +22,6 @@ class RandomNumberGenerator;
 
 namespace internal {
 
-class SamplingAllocationObserver;
-
 class AllocationProfile : public v8::AllocationProfile {
  public:
   AllocationProfile() = default;