arthur-tf-sync-repos: remove unused credential

Signed-off-by: Arthur She <arthur.she@linaro.org>
Change-Id: I03404bfb43a46de2cc9459520f46294713976d27
diff --git a/arthur-tf-sync-repos.yaml b/arthur-tf-sync-repos.yaml
index 20a8872..bdc7df0 100644
--- a/arthur-tf-sync-repos.yaml
+++ b/arthur-tf-sync-repos.yaml
@@ -61,7 +61,3 @@
       - text:
           credential-id: openci-bot-github-token
           variable: AUTH_TOKEN
-      - text:
-          credential-id: Trusted_Firmware_ReadTheDocs_Token
-          variable: RTD_TOKEN
-