Revert "Also retry on transaction serialization errors (#8317)" (#8334)

TransactionSerializationError is a subtype of TransactionConflictError.
This PR was a no-op.

This reverts commit e6a7994.

edb/testbase/connection.py

@@ -416,10 +416,7 @@ async def _retry_operation(self, func):
             # since that *ought* to be done at the transaction level.
             # Though in reality in the test suite it is usually done at the
             # test runner level.
-            except (
-                errors.TransactionConflictError,
-                errors.TransactionSerializationError,
-            ):
+            except errors.TransactionConflictError:
                 if i >= 10 or self.is_in_transaction():
                     raise
                 await asyncio.sleep(