Move MetricsTaskServlet runtime dep to backend

This is a follow-up to [] that corrects an oversight.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=119683462

java/com/google/domain/registry/module/backend/BUILD

@@ -48,5 +48,6 @@ java_binary(
     create_executable = 0,
     runtime_deps = [
         ":backend",
+        "//java/com/google/domain/registry/monitoring/whitebox",  # MetricsTaskServlet
     ],
 )

java/com/google/domain/registry/module/frontend/BUILD

@@ -36,7 +36,6 @@ java_binary(
     create_executable = 0,
     runtime_deps = [
         ":frontend",
-        "//java/com/google/domain/registry/monitoring/whitebox",  # MetricsTaskServlet
         "//java/com/google/domain/registry/ui/server/api",  # CheckApiServlet
         "//java/com/google/domain/registry/ui/server/registrar",  # ResourceServlet
     ],