Update CODEOWNERS

Keep frontend only

Author: academo

.github/CODEOWNERS

@@ -1,4 +1,4 @@
 # These owners will be the default owners for everything in
 # the repo. Unless a later match takes precedence,
 # these will be requested for review when someone opens a pull request.
-* @grafana/plugins-platform-frontend @grafana/plugins-platform-backend
+* @grafana/plugins-platform-frontend