[launcher] Remove outdated TODO.

The `RankingItemType` has been removed in the previous CL (https://chromium-review.googlesource.com/c/chromium/src/+/4054720), and `launched_from` is still in active use (https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/ui/app_list/search/ranking/mrfu_ranker.cc;drc=2899fd9a2767ef605edff106d1e00ff45af08a2e;l=45). Thus, this TODO is outdated and should be removed.

Bug: b/259481518, b/259448456
Change-Id: I3c847534ac4ede6b00489ca6affe3e5ead08719a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4060519
Auto-Submit: CJ Huang <chenjih@google.com>
Commit-Queue: Rachel Wong <wrong@chromium.org>
Reviewed-by: Rachel Wong <wrong@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1077216}