Strip unknown fields from newly added fields in PerfDataProto

This clears unused fields from newly added event_type and pmu_mappings
fields, and also unknown fields from the root level of PerfDataProto.

A follow-up change will also remove unused types of events, after I
port the change to align the event types.

BUG=b:233252585
TEST=unit_tests

Change-Id: I6f6f9c3b0ef1815d619897d3ab9e05f8000bcf90
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3696708
Commit-Queue: Tuo Shan <shantuo@google.com>
Reviewed-by: Gabriel Marin <gmx@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1029358}