Remove UI/IOThreadExecutor used for base::CurrentThread

Per https://crrev.com/c/1978821, base::CurrentThread concept
was deprecated in Tasks API v3. The cleanup was not fully completed.
UI/IOThreadExecutor were introduced to support base::CurrentThread and
thus are now obsolete. The corresponding tests had already
been deleted.

Doing this cleanup because I need to make changes to this code
and the current state would complicate my design.

Bug: 40108370
Change-Id: I230ef59c930ead2e90e04b24d2f88a58f28b4687
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6163424
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Chidera Olibie <colibie@google.com>
Cr-Commit-Position: refs/heads/main@{#1408959}