Revert "[vm/ffi] Enable creating an ExternalTypedData from a Pointer."

This reverts commit ddd83d256f2423ad4218bf5fee154977ff1e2e55.

Reason for revert: Causing build failures in the Flutter engine, blocking the SDK roll. Build error:

org-dartlang-sdk:///third_party/dart/runtime/lib/ffi_patch.dart:76:3: Error: Type 'TypedData' not found.
  TypedData asExternalTypedData({int count: 1}) =>
  ^^^^^^^^^



Original change's description:
> [vm/ffi] Enable creating an ExternalTypedData from a Pointer.
> 
> Fixes dartbug.com/37738
> 
> Change-Id: I65c6741978d36cd1c255039a4dd8a06190ea4366
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/111736
> Commit-Queue: Samir Jindel <sjindel@google.com>
> Reviewed-by: Daco Harkes <dacoharkes@google.com>

TBR=vegorov@google.com,sjindel@google.com,dacoharkes@google.com

Change-Id: I90a9cb5e2894c0bc064e620cef0a0f51505ac05d
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/112047
Reviewed-by: Ben Konyi <bkonyi@google.com>
Commit-Queue: Ben Konyi <bkonyi@google.com>