[omnibox] Make the AI Mode placeholder text translateable.

In crrev.com/c/6842653, we marked the placeholder text as
translateable="false" in order to make it safely mergeable to the
release branch. Going forward, we'll need translations for this string
so this removes the annotation.

The earlier CL already uploaded a screenshot and checked in the hash:
https://source.chromium.org/chromium/chromium/src/+/main:chrome/app/generated_resources_grd/IDS_OMNIBOX_AIM_PLACEHOLDER_TEXT.png.sha1

Skip-Translation-Screenshots-Check: True
Bug: 441310305
Change-Id: Id973b5d266505b246c5aa90adb959cc954ba541b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6892227
Reviewed-by: Khalid Peer <khalidpeer@chromium.org>
Commit-Queue: Justin Donnelly <jdonnelly@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1507800}