I don't know how to completely remove them. You can sort of minimise them...
If you convert your grouping to text mode, the lines that control what appears here are (in this example grouping by project name).
group.0.namekeyargkey.0=project group.0.namekeyargkey.1=name
If you remove the lines completely (or remove everything after the = sign), you get {0}: {1}: as your prefix which isn't great.
It doesn't work to blank it out using a space or quotes. The most discreet option I can recommned is to use a full stop.
group.0.namekeyargkey.0=. group.0.namekeyargkey.1=.
You'll still get .: .: but it's certainly not as obtrusive.