[cr-buildbucket.cfg] Remove defunct luci_migrator_host entry.

This field is deprecated and has no effect:
https://chromium.googlesource.com/infra/luci/luci-go/+/master/buildbucket/proto/project_config.proto#423

Change-Id: I04237100342e408972cad120ae70c8909585f2c4
diff --git a/tools/luci/config/cr-buildbucket.cfg b/tools/luci/config/cr-buildbucket.cfg
index 89aeda2..68dcc97 100644
--- a/tools/luci/config/cr-buildbucket.cfg
+++ b/tools/luci/config/cr-buildbucket.cfg
@@ -41,7 +41,6 @@
       build_numbers: YES
       # Some builders require specific hardware, so we make the assignment in bots.cfg
       auto_builder_dimension: YES
-      luci_migration_host: "luci-migration.appspot.com"
       recipe {
         cipd_package: "infra/recipe_bundles/chromium.googlesource.com/chromium/tools/build"
         cipd_version: "refs/heads/master"