[PA] Remove unused ref_counted / scoped_refptr / AtomicRefCount

An include/usage census of partition_alloc_base shows that
memory/ref_counted.{h,cc}, memory/scoped_refptr.h and
atomic_ref_count.h have no users anywhere in PartitionAlloc
(production or tests), nor in the few external files that include
partition_alloc_base headers. ref_counted.cc was still being compiled
into allocator_base. Delete the cluster (~680 LoC) and drop the
BUILD.gn entries.

Bug: none
Change-Id: I8e221f38f43562d7fabd396e4c55e6e831870728
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8586556
Auto-Submit: Mikihito Matsuura <mikt@google.com>
Reviewed-by: Keishi Hattori <keishi@chromium.org>
Commit-Queue: Keishi Hattori <keishi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1719718}
NOKEYCHECK=True
GitOrigin-RevId: 37f66d968e7bff7e3dfcfa44f0aee7877dee1334