chore: remove old native image kokoro downstream jobs (#2629)

Fixes #2478
Removing in favor of newly introduced [cloudbuild downstream presubmit
jobs.
](https://github.com/googleapis/sdk-platform-java/tree/main/.cloudbuild/graalvm)

.kokoro/presubmit/common.cfg

@@ -30,12 +30,4 @@ env_vars: {
 env_vars: {
   key: "SECRET_MANAGER_KEYS"
   value: "cloud-java-ci-it-service-account"
-}
-
-# Defines the google-cloud-java modules to be tested downstream tested with GraalVM native image
-# builds.
-env_vars: {
-  key: "MODULES_UNDER_TEST"
-  value: "java-kms,java-os-login" # KMS's ITKmsTest exercises gax-java's native image
-  # reflect-config.json. OS Login has a similar issue.
-}
+}