Cleanup: remove unnecessary TODOs.

In addition to removing TODOs, I have also revised the switch
to make the compiler detect unknown fetch handler type instead.

Bug: 1347319, 1351246
Change-Id: I11ec5ad8f16ffe4aa334b25a3bde8c222216d187
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5086559
Reviewed-by: Shunya Shishido <sisidovski@chromium.org>
Commit-Queue: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Reviewed-by: Minoru Chikamune <chikamune@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1233730}