Set default From address in config (#3756)

The old implementation sets default From address in mailers. It sets
the address from SMTP_FROM_ADDRESS, or notifications@localhost. The
behavior is occasionally undesired results.

In production environment, notifications@localhost is likely to be
incorrect.
In testing environment, the email address should not be varied by a
environment variable.

After appling this change,

In production environment, it will throw an exception when launching
Mastodon.
In testing environment, the address will be fixed with
notifications@localhost.

app/mailers/application_mailer.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 class ApplicationMailer < ActionMailer::Base
-  default from: ENV.fetch('SMTP_FROM_ADDRESS') { 'notifications@localhost' }
   layout 'mailer'
   helper :instance
 

app/mailers/user_mailer.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 class UserMailer < Devise::Mailer
-  default from: ENV.fetch('SMTP_FROM_ADDRESS') { 'notifications@localhost' }
   layout 'mailer'
 
   helper :instance