Translator: Remove deprecated window.translation

The window.translation API has been deprecated in favor of
window.ai.translator API.

Besides chrome/browser/policy/test/translator_api_policy_browsertest.cc,
this just removes code associated with window.translation.
translator_api_policy_browsertest.cc was missed when updating
browsertests to use window.ai.translator.

Fixed: 392185835
Change-Id: I3ef11db634a87fdd572cf57593bd0674b18afe8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6227852
Reviewed-by: Alexei Svitkine <[email protected]>
Reviewed-by: Ming-Ying Chung <[email protected]>
Reviewed-by: Daniel Cheng <[email protected]>
Reviewed-by: Sergey Poromov <[email protected]>
Commit-Queue: Nathan Memmott <[email protected]>
Reviewed-by: Domenic Denicola <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1426365}

Author: Nathan Memmott