CiviMail: Do not default subject field to mailing name

From U.S. PIRG comments:

Subject should not default to the name of the mailing. The name of the mailing is a string which helps us stay organized internally. The subject line is the most important component of the public "face" of the e-mail. They serve wildly different purposes and should not be the same. We've already inadvertently sent a couple mailings with the name in the subject, which looks weird.