Refs #35859 -- Removed support for Task enqueuing on transaction commit.

This removes the ability to configure Task enqueueing via a setting,
since the proposed `ENQUEUE_ON_COMMIT` did not support multi-database
setups.

Thanks to Simon Charette for the report.

Follow-up to 4289966d1b.

tests/tasks/test_custom_backend.py

@@ -27,7 +27,6 @@ class CustomBackendNoEnqueue(BaseTaskBackend):
     TASKS={
         "default": {
             "BACKEND": f"{CustomBackend.__module__}.{CustomBackend.__qualname__}",
-            "ENQUEUE_ON_COMMIT": False,
             "OPTIONS": {"prefix": "PREFIX: "},
         },
         "no_enqueue": {