Reset the campaigns previews

I had used a debug image which I left in and which looks broken.
Removing it defaults to the normal 'missing preview' 0 A.D. image which
looks more correct.

Differential Revision: https://code.wildfiregames.com/D4172
This was SVN commit r25801.

binaries/data/mods/public/campaigns/new_maps.json

@@ -1,7 +1,6 @@
 {
 	"Name": "Demo campaign - new maps",
 	"Description": "Discover the new maps in Alpha 25 with a demo campaign, taking you through all of them.",
-	"Image": "session/icons/mappreview/Introductory_Tutorial.png",
 	"Levels": {
 		"alpine_valleys_2p": {
 			"Map": "skirmishes/alpine_valleys_2p.xml",

binaries/data/mods/public/campaigns/tutorial.json

@@ -1,7 +1,6 @@
 {
 	"Name": "Tutorial",
 	"Description": "Learn how to play 0 A.D.",
-	"Image": "session/icons/mappreview/Introductory_Tutorial.png",
 	"Levels": {
 		"introduction": {
 			"Name": "Introductory Tutorial",