dropNthFrame.jsx

//Script drops every Nth frame starting from a given frame
//by placing TimeRemap keyframes
//CC-BY Nik Ska
//2014-2016

var dropNthFrame = function (frameNum, startFrame) {

    var activeItem = app.project.activeItem;
    if(activeItem != null && activeItem instanceof CompItem){
        
        var startFrame = startFrame || 0; //when to start
        var delta = frameNum; //Drop this frame
        var fd = activeItem.frameDuration;

        var sel = activeItem.selectedLayers;
        if(sel.length > 0){

            app.beginUndoGroup("Dropping every " + String(delta+1) + " frame");

            if(sel[0].timeRemapEnabled == false){ 
                sel[0].timeRemapEnabled = true;
            }

            delta--;

            var curFrame = startFrame*fd;
            var tr = sel[0].property("ADBE Time Remapping");
            var shift = 0;
            
            while((curFrame+fd*(delta))<sel[0].source.duration){
                tr.setValueAtKey(tr.addKey(curFrame-shift*fd), curFrame); //add first
                tr.setValueAtKey(tr.addKey(curFrame+fd*(delta-shift)), curFrame+delta*fd); //add second +shift
                curFrame += (delta*fd + 2*fd); //increment frame
                shift++;
            }

            app.endUndoGroup();

        }
    }
}

dropNthFrame(5,0);