[compiler] Cleanup owners file

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}

src/compiler/OWNERS

@@ -10,10 +10,6 @@ tebbi@chromium.org
 neis@chromium.org
 mvstanton@chromium.org
 
-# For backend
-bbudge@chromium.org
-gdeepti@chromium.org
-
 per-file wasm-*=ahaas@chromium.org
 per-file wasm-*=binji@chromium.org
 per-file wasm-*=clemensh@chromium.org

src/compiler/backend/OWNERS

 bbudge@chromium.org
 gdeepti@chromium.org
 
+# Plus src/compiler owners.
+
 # COMPONENT: Blink>JavaScript>Compiler