Remove optimization_guide:: namespace from hints_manager*

Not needed since it is already in that namespace

Change-Id: I85ae3e133760f2977df0f7e0269ea61ddb1efb9f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3319420
Auto-Submit: Sophie Chang <sophiechang@chromium.org>
Reviewed-by: rajendrant <rajendrant@chromium.org>
Commit-Queue: rajendrant <rajendrant@chromium.org>
Cr-Commit-Position: refs/heads/main@{#949096}