Reverting because it feels backward to specify a delivery to not
be performed while the e-mail is being composed. It is simpler (and
makes more sense) to delegate the responsibility to the calling code.

This reverts commit fa6d921e.

Reason: Not a fan of such massive changes. We usually close such changes
if made to Rails master as a pull request. Following the same principle
here and reverting.

[ci skip]

For future reference, this is the regex I used: ^\s*#\s*\n(?!\s*#). Replace
with the first match, and voilà! Note that the regex matches a little bit too
much, so you probably want to `git add -i .` and go through every single diff
to check if it actually should be changed.

one.

Added delivery_handler method to mail and implemented in ActionMailer to deliver inside of instrumentation