Backend is split out, so remove backend owners from src/compiler. Also,
mention explicitly in src/compiler/backend that also all src/compiler
owners are owners there.

R=titzer@chromium.org

No-Try: true
Change-Id: I5409946f65bf27337b715af555083a4804fbb8dd
Reviewed-on: https://chromium-review.googlesource.com/c/1400411Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58626}