Remove transaction from QuerySet.update_or_create's save()

[Tim Graham]

According to Aymeric on ​the pull request:

It will have beneficial performance effects if:

• ATOMIC_REQUESTS is not in use
• update_or_create ends up creating an object
• the save method of that objects does long things like sending email after the write (i.e. once it holds an exclusive lock on the database)

[Tim Graham <timograham@…>]

In 423b3afc:

Fixed #25939 -- Removed redundant transaction in QuerySet.update_or_create().

There is no need to wrap the save() call in transaction.atomic() as
it's already done down the call stack in Model.save_base().