This reverts commit 650d30d8.

Some mailing lists are configured add prefix "[listname] " to all their
messages, and also people hand-edit subject lines, be it an output from
format-patch or a patch generated by some other means.

We cannot stop people from mucking with the subject line, and with the
change, there always will be need for hand editing the subject when that
happens.  People have depended on the leading [bracketed string] removal.