Unused option subjectprefix still in use

[Nazgul]

During the upgrade process a few unused options are removed, among them subjectprefix.

But this option is still in use in wp-mail.php:

$subject = trim(str_replace(get_option('subjectprefix'), '', $subject));

Because this results in "" being replaced by "" it has no ill effects, but I suggest removing this (now) useless reference anyway.

[markjaquith]

(In [6056]) Skip deprecated option replacement in wp-mail.php. props Nazgul. fixes #4924