cmd/trace: collapse v2 directory into trace

cmd/trace: collapse v2 directory into trace

This change removes the old trace code and replaces it with the new tracer.
It does the following:
- Moves the contents of the v2 directory into the parent trace directory.
- Combines the old tracer main file with the new main file.
- Replaces any existing files with the corresponding v2 files.
- Removes any unused files.

Updates #67367

Change-Id: I2237920e13588258a2442b639d562cf7f8a8e944
Reviewed-on: https://go-review.googlesource.com/c/go/+/584536
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>