Change autocomment-prefix to be blank. Looks weird being '- '

Change autocomment-prefix to be blank. Looks weird being '- '

This messages inserts stuff before the special formatting in
/* header */ comments, if it is not the first thing in the
edit summary. I'm assuming putting a dash there made sense
originally, but now with the → it looks kind of weird imo.
Also it can be confused since '-' is often taken to mean
removal of content.

I was considering using '-' to disable the message as is normally
done, but in this case - is a reasonable value (and pretty close
to the old value).

Request by accolade on irc

Change-Id: I2622b35e712120dc171ea44a887d6b42286f2a06