chore: Stop clobbering KOKORO_KEYSTORE_DIR (#204)

Author: dazuma

.trampolinerc

@@ -21,7 +21,6 @@ pass_down_envvars+=(
     "AUTORELEASE_PR"
     "EXTRA_CI_ARGS"
     "KOKORO_GIT_COMMIT"
-    "KOKORO_KEYSTORE_DIR"
     "RELEASE_DRY_RUN"
     "RELEASE_PACKAGE"
     "RUBY_VERSIONS"