Remove rfc-tool from builder dispatch matrix

The ponylang/rfc-tool repo has been archived.

Author: SeanTAllen

.github/workflows/rebuild-ponyc-based-images.yml

@@ -708,7 +708,6 @@ jobs:
           - ponylang/reactive_streams
           - ponylang/redis
           - ponylang/regex
-          - ponylang/rfc-tool
           - ponylang/semver
           - ponylang/ssl
           - ponylang/templates