Fix delete_where() leaving the connection in an open transaction

Table.delete_where() ran its DELETE via a bare execute() with no
commit, unlike Table.delete() which wraps in db.atomic(). The
connection was left with an open implicit transaction, so the
deletion (and all subsequent writes, including later atomic()
blocks which switched to savepoint mode) was silently rolled back
when the connection closed.

Wrap the DELETE in db.atomic() and remove the now-unnecessary
atomic() wrapper from the delete_where documentation example.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UnLnhsH25Nnv7LHhekUfPd

docs/python-api.rst

@@ -984,8 +984,7 @@ You can delete all records in a table that match a specific WHERE statement usin
 
     >>> db = sqlite_utils.Database("dogs.db")
     >>> # Delete every dog with age less than 3
-    >>> with db.atomic():
-    >>>     db.table("dogs").delete_where("age < ?", [3])
+    >>> db.table("dogs").delete_where("age < ?", [3])
 
 Calling ``table.delete_where()`` with no other arguments will delete every row in the table.