This CL removes serialization of JSTypedArray fields when direct heap
reads are enabled. Invariants we rely on:

- Of the underlying interesting fields,
  - base_pointer and external_pointer are set either during
    initialization, or in a one-time on-to-off-heap transition in
    GetBuffer.
  - length and buffer are immutable after initialization.
- is_on_heap and DataPtr derive from base_pointer and
  external_pointer s.t. is_on_heap == (base_pointer != 0) and
  DataPtr == external_pointer in the off-heap case.

In this CL we add one new invariant:

- For all base_pointer and external_pointer mutations after
  initialization, base_pointer is guaranteed to be release-stored
  after external_pointer has been written.

With these invariants, concurrent access to off-heap typed arrays is
trivial as long as is_on_heap (= base_pointer) is read before other
relevant fields.

Note that JSTypedArray remains a kSerializedHeapObject due to the
serialized superclass JSObject.

Drive-by: Remove unused Torque operators and empty TODOs.

Bug: v8:7790
Change-Id: I3c4327318f94e4e6083d4e87476069aad2649386
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_isolates_rel_ng
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2679689
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72572}