[attribution_reporting] Remove [[nodiscard]] from StorableTrigger getters

See https://chromium-review.googlesource.com/c/chromium/src/+/3358509/14/content/browser/attribution_reporting/attribution_report.h#107 for similar rationale

Change-Id: I45c3d1e4399a91887f03f107100fbdf1404f5324
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3388878


Reviewed-by: Andrew Paseltiner <apaseltiner@chromium.org>
Commit-Queue: Andrew Paseltiner <apaseltiner@chromium.org>
Cr-Commit-Position: refs/heads/main@{#960146}