Fix get_available_rmw_implementations() implementation.

Signed-off-by: Michel Hidalgo <michel@ekumenlabs.com>

rmw_implementation/rmw_implementation/__init__.py

@@ -20,7 +20,7 @@
 def get_available_rmw_implementations():
     """Return a list of all available RMW implementations as registered in the ament index."""
     rmw_implementations = ament_index_python.get_resources('rmw_typesupport')
-    return [name for name, _ in rmw_implementations if name != 'rmw_implementation']
+    return [name for name in rmw_implementations if name != 'rmw_implementation']
 
 
 __all__ = [