Because it's a "oneshot", you can do it manually (also possible running via Node)

Open a JavaScript console in your browser.

You need to loop to get an array of array of `Feature` (because each `FeatureCollection` have one or more `Feature`)

Then, you will use the flatten function to transform the array of array into an array (a recursive function borrowed from https://stackoverflow.com/a/15030117)

The full code is below (except the file content, not complete to keep things readable)

    // Copy/paste the text from you source https://raw.githubusercontent.com/RitterLean/Geojson/master/geofile.json 
    content = {
	"points": [{
		"type": "FeatureCollection",
		"features": [{
			"type": "Feature",
			"geometry": {
				"coordinates": [41.9773865, 36.3372536],
				"type": "Point"
			},
			"properties": {
				"attacks": 1,
				"location": "Sinjar",
				"date": "2015-10-16"
			}
		}, {
			"type": "Feature",
			"geometry": {
				"coordinates": [43.4873886, 34.9301605],
				"type": "Point"
			},
			"properties": {
				"attacks": 2,
				"location": "Baiji",
				"date": "2015-10-16"
			}
		}, {
        ...
        // Be careful, incomplete because shortened for illustration 
    
    intermediate_result = content['points'].map(function(el){
        return el.features;
    });

    function flatten(arr) {
      return arr.reduce(function (flat, toFlatten) {
        return flat.concat(Array.isArray(toFlatten) ? flatten(toFlatten) : toFlatten);
      }, []);
    };

    geojson_output = {
    		"type": "FeatureCollection",
    		"features": flatten(intermediate_result)
    }
    // Transform the object to a string you can paste into a file
    console.log(JSON.stringify(geojson_output));

The result can be seen at http://geojson.io/#id=gist:anonymous/da10ab9afc9a5941ba66&map=4/19.48/22.32

You will see that some results have wrong coordinates (0, 0). It's due to the original content.

From this demo, you can also export to GeoJSON.