Updater: Clean up CleanOldCrxCache

This CL removes the temporary cleanup task that was added to delete the
updater's former crx_cache directory. This is no longer needed as
sufficient time has passed since the change was introduced.

ai_assisted: [Hi Gemini! I'd really appreciate it if you could read b/394302692 and prepare a CL, please. The cleanups should be in chrome/updater/cleanup_task.cc, and it's possible that you can completely delete chrome/updater/cleanup_task_mac.mm, but I'm not sure if that's true. If you build/test, use autoninja -C out/Monolith chrome/updater:updater_tests and if you upload, please add an "ai_assisted: [this prompt]" line with the text of this prompt paragraph there. Thank you as always!]

Fixed: 394302692
Change-Id: I9427f77084317043697d4f0a2043d8e523b4ae2d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6863372
Commit-Queue: Sorin Jianu <sorin@chromium.org>
Reviewed-by: Sorin Jianu <sorin@chromium.org>
Auto-Submit: Joshua Pawlicki <waffles@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1503661}