Remove section on breaking the mailer tests

As far as I can tell, making the suggested changes doesn't actually
break the test suite. I tried this with the current version of the
project as well as a version from back in August.
Remove references to previously removed section
Remove reference to Emails in OrderControllers

As it stands implementing the instructions in the tutorial, there
doesn’t seem to be any additional emails sent as opposed to when we
modified `mailer.rb`.
Change user information in User#create example

Using different user information helps make it clear that this test is
sending additional email. Otherwise, it’s hard to tell the difference
from before we added this example.
Adjust parameter due to removed local variable
Add a note to run the specs

It’s not out of the question that the reader may have made a mistake
implementing this change, so now is a good time to run the specs and
make sure everything is gravy.
Remove duplicate instruction

We removed the direct access to Mailer in earlier in the tutorial.
Simplify failing tests

The aforementioned tests should fail, but the exact stack traces may
vary depending on implementation.
Simplify Redis setup

Redis no long takes a path to a database. I think we should get it
working, but as it stands, it adds a bit of complexity that isn’t
helping the core understanding.
Clarify why they should be receiving double emails
Remove Redis ports for now

See 4f577cc for more information
Merge pull request #1003 from JumpstartLab/extract-notifications-update

Extract notifications update