[AIRFLOW-2798] Remove needless code from models.py
[AIRFLOW-2798] Remove needless code from models.py

If `email_on_failure` or `email_on_failure` set to
TRUE and email available, there will be email sent
out on event of retry or failure.

In the implementation, there is a argument
`is_retry` passed to method `self.email_alert`.
However, inside this method, this argument is not
used at all. I believer the initial author of this
method was planning to differentiate the email to
be sent out, but for whatever reason this was not
implemented.

Given in the email to be sent out, there will be a
line "Try {try_number} out of {max_tries}<br>", it
would be fine not to differentiate **retry email**
and **failure email**.

Closes #3640 from XD-DENG/patch-1

lxneng/incubator-airflow,lxneng/incubator-airflow,lxneng/incubator-airflow,lxneng/incubator-airflow