Remove future extensions from ARCHITECTURE.md

Removed future extensions section from architecture document.

Author: simbo1905

json-java21-jtd/ARCHITECTURE.md

@@ -307,11 +307,3 @@ This implementation strictly follows RFC 8927:
 - ✅ Definition support (non-nested)
 - ✅ Timestamp format validation (RFC 3339)
 - ✅ No remote schema support (simplification by design)
-
-## Future Extensions
-
-Potential future additions (non-RFC compliant):
-- Custom type validators
-- Additional format validators  
-- Remote definition support
-- Performance optimizations for specific use cases