Revert "[vm] Generate event ids for synchronous begin and end events."

This reverts commit 7fe44864d6b8e39eb1034de388f4d373ced2b6ee.

Reason for revert: https://github.com/flutter/flutter/issues/106957

Original change's description:
> [vm] Generate event ids for synchronous begin and end events.
>
> os_signposts needs explicit ids to match begin and end events, whereas all other trace systems do this implicitly.
>
> TEST=Instruments
> Bug: https://github.com/dart-lang/sdk/issues/49178
> Change-Id: Ifeeaa1dfe823a0d5668f48b8c584f8bdeef23212
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/249951
> Reviewed-by: Siva Annamalai <asiva@google.com>
> Commit-Queue: Ryan Macnak <rmacnak@google.com>

TBR=rmacnak@google.com,asiva@google.com

Change-Id: Id86959d08b5352b5446b6f294a8302b88b4fa0c8
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: https://github.com/dart-lang/sdk/issues/49178
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/250403
Reviewed-by: Zach Anderson <zra@google.com>
Commit-Queue: Liam Appelbe <liama@google.com>
Reviewed-by: Siva Annamalai <asiva@google.com>
Reviewed-by: Liam Appelbe <liama@google.com>