Fix C++20 compilation

Several std::allocator<> members were deprecated in C++17 and
subsequently removed in C++20. Drop deprecated members altogether since
these are unused anyway.

Change-Id: Ic26471a4b1cb3ccc1fdf3231400c81827b0ff712

internal/ceres/fixed_array_test.cc

diff --git a/internal/ceres/fixed_array_test.cc b/internal/ceres/fixed_array_test.cc
index d418786..f29f809 100644
--- a/internal/ceres/fixed_array_test.cc
+++ b/internal/ceres/fixed_array_test.cc
@@ -655,7 +655,6 @@
 class CountingAllocator : public std::allocator<T> {
  public:
   using Alloc = std::allocator<T>;
-  using pointer = typename Alloc::pointer;
   using size_type = typename Alloc::size_type;
 
   CountingAllocator() : bytes_used_(nullptr), instance_count_(nullptr) {}
@@ -670,39 +669,18 @@
         bytes_used_(x.bytes_used_),
         instance_count_(x.instance_count_) {}
 
-  pointer allocate(size_type n, const void* const hint = nullptr) {
+  T* allocate(size_type n) {
     assert(bytes_used_ != nullptr);
     *bytes_used_ += n * sizeof(T);
-    return Alloc::allocate(n, hint);
+    return Alloc::allocate(n);
   }
 
-  void deallocate(pointer p, size_type n) {
+  void deallocate(T* p, size_type n) {
     Alloc::deallocate(p, n);
     assert(bytes_used_ != nullptr);
     *bytes_used_ -= n * sizeof(T);
   }
 
-  template <typename... Args>
-  void construct(pointer p, Args&&... args) {
-    Alloc::construct(p, std::forward<Args>(args)...);
-    if (instance_count_) {
-      *instance_count_ += 1;
-    }
-  }
-
-  void destroy(pointer p) {
-    Alloc::destroy(p);
-    if (instance_count_) {
-      *instance_count_ -= 1;
-    }
-  }
-
-  template <typename U>
-  class rebind {
-   public:
-    using other = CountingAllocator<U>;
-  };
-
   int64_t* bytes_used_;
   int64_t* instance_count_;
 };