[Code Health] Remove some expired Mojo.Channel.* histograms

OBSOLETE_HISTOGRAM[Mojo.Channel.WriteMessageHandles]=Expired in 2021
OBSOLETE_HISTOGRAM[Mojo.Channel.WritevBatchedMessages]=Expired in 2022

Bug: 1447228
Change-Id: I3ad40316098868e7b96f9a9c69bdac7e596a3b7a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4582791


Reviewed-by: Brian Geffon <bgeffon@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Oksana Zhuravlova <oksamyt@chromium.org>
Reviewed-by: Weilun Shi <sweilun@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1153878}