A few low-risk cleanups found while combing through the extension:

- Remove the unused `getTimestampString()` from `src/utils.ts`. It's been
  dead since 2017 (`1b424000`), when "Add timestamps and log levels to all
  extension-side log messages" moved timestamping into the `Logger` and
  deleted its three call sites. It also had a latent bug: no zero-padding,
  so it rendered times like `[9:5:3]`.
- Remove the unused `getWindowsSystemPowerShellPath()` from
  `src/platform.ts`. It's been dead since #2238 (2019), which introduced
  `PowerShellExeFinder` and deleted the `System32PowerShellPath` /
  `SysnativePowerShellPath` / `SysWow64PowerShellPath` constants that
  called it. The same path is now built inline in `findWinPS()`, with
  proper bitness handling in `getSystem32Path()`.
- Replace `console.log` with `this.logger.writeDebug` in
  `writePidIfInDevMode` (`src/session.ts`), per our `ILogger` convention.
  While here, `await` the `fs.delete` it sits next to so the "Deleted PID
  file" message is actually true — previously it was a floating promise and
  the log ran before the delete resolved.
- Point the PSScriptAnalyzer rule docs link at `learn.microsoft.com`
  instead of `docs.microsoft.com` (`src/features/CodeActions.ts`); the old
  URL just 301s there anyway.

I left the Command Explorer's dead code alone since #5508 already rewrites
that file. Compile, lint, and format all pass; the touched code has no test
coverage so risk is low.

Drafted by Copilot (Claude Opus 4.8).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>