This shows up if you have saved reports. So just remove the non primary reports.
If you select the report and then go under Actions-Reports-Delete report.
That should remove the currently selected report. If there are no extra reports, only the primary one, then this shouldnt show up.
If you really only just want to hide this one field. Play around in the console and once you have it set you can have the code run sometime after load, maybe add in a wait or something so you can be sure the IG has already loaded.
I did the following(emp is the IG static ID):
var ig$ = apex.region("emp").widget();
var temp = ig$.interactiveGrid("getToolbar")[0]
temp.children[0].children[1].hidden = true
If you play around in the console with this you will see what is selected each time as you go a layer deeper. Then finaly we get just to the select list of the Reports, we get its property hidden which is false by default, we change it to true.
So now its hidden.
This methodology works for damn near everything. In the console you find a way to select the region or something inside it that includes whatever it is you want. Then go deeper and deeper until you are only selecting the specific thing you want. Then you can hide it, change it, get its value, id,...