It's surprising that there is no way to configure this. You can choose whether the Author: header should appear, but Summary: is jammed in by default no matter what.
I had to redefine a function:
(defun log-edit-insert-message-template ()
"Insert the default template."
(when (or (called-interactively-p 'interactive)
(insert "\nAuthor: "))
Compared with the vanilla version in log-edit.el, I deleted the lines adding "Summary:" and "\n\n".
log-edit first is needed (if you put the above code in your
.emacs) because otherwise the new definition would be overwritten when that library is loaded the first time you do a commit operation.