Remove TaskFail duplicates check (apache#26714)

This check was added in error.  The duplicates are supposed to be there.  The task duration
and gantt views process TaskFail records with the apparent intention of including durations
from non-final failed attempts to the overall duration of the run.

(cherry picked from commit ee21c1b)

airflow/utils/db.py

@@ -926,29 +926,31 @@ def reflect_tables(tables: Optional[List[Union[Base, str]]], session):
     return metadata
 
 
-def check_task_fail_for_duplicates(session):
-    """Check that there are no duplicates in the task_fail table before creating FK"""
-    metadata = reflect_tables([TaskFail], session)
-    task_fail = metadata.tables.get(TaskFail.__tablename__)  # type: ignore
-    if task_fail is None:  # table not there
-        return
-    if "run_id" in task_fail.columns:  # upgrade already applied
-        return
-    yield from check_table_for_duplicates(
-        table_name=task_fail.name,
-        uniqueness=['dag_id', 'task_id', 'execution_date'],
-        session=session,
-        version='2.3',
-    )
-
-
 def check_table_for_duplicates(
     *, session: Session, table_name: str, uniqueness: List[str], version: str
 ) -> Iterable[str]:
     """
     Check table for duplicates, given a list of columns which define the uniqueness of the table.
 
-    Call from ``run_duplicates_checks``.
+    Usage example:
+
+    .. code-block:: python
+
+        def check_task_fail_for_duplicates(session):
+            from airflow.models.taskfail import TaskFail
+
+            metadata = reflect_tables([TaskFail], session)
+            task_fail = metadata.tables.get(TaskFail.__tablename__)  # type: ignore
+            if task_fail is None:  # table not there
+                return
+            if "run_id" in task_fail.columns:  # upgrade already applied
+                return
+            yield from check_table_for_duplicates(
+                table_name=task_fail.name,
+                uniqueness=['dag_id', 'task_id', 'execution_date'],
+                session=session,
+                version='2.3',
+            )
 
     :param table_name: table name to check
     :param uniqueness: uniqueness constraint to evaluate against
@@ -1393,7 +1395,6 @@ def _check_migration_errors(session: Session = NEW_SESSION) -> Iterable[str]:
     :rtype: list[str]
     """
     check_functions: Tuple[Callable[..., Iterable[str]], ...] = (
-        check_task_fail_for_duplicates,
         check_conn_id_duplicates,
         check_conn_type_null,
         check_run_id_null,