I can't find how to remove the Group Headers in KMail.

I have a folder sorted by date. I don't need the "Day" group header. In a threaded folder with few threads/messages the Group Headers are wasting valuable space in a list with only 17 "lines" in view.

To give you an idea here's the top of the message list:
Header Thread Header Thread
Header Thread Thread
Header Header Header Header
A bunch of older Threads

See what I mean?

The best I can do is to edit the Theme to remove the text. That reduces the size of the Group Header but I can't remove it.

Can anyone help me?

"Select Aggregation Mode" button ( 4th along from search bar ) > Configure. You can then create a customised view how you would want it to be. Make sure to activate it the folder.

The OP probably meant to get an uncluttered view without modifying the theme - which I prefer as well.

Steps would be changing the theme to Classic and to disable grouping change the aggregation to Standard Mailing List.

See http://userbase.kde.org/KMail/FAQs_Hints_and_Tips#Get_more_screen_space_in_KMail