Drop ifvisible as it was only used for Prefetch and that feature was removed in v2.17

dev/External/ifvisible.js

@@ -1,40 +0,0 @@
-(doc => {
-	let idle = 'idle',
-		visible = 'visible',
-		status = visible,
-		timer = 0,
-		wakeUp = () => {
-			clearTimeout(timer);
-			status = visible;
-			timer = setTimeout(() => {
-				if (status === visible) {
-					status = idle;
-					dispatchEvent(new CustomEvent(idle));
-				}
-			}, 10000);
-		},
-		init = () => {
-			init = () => 0;
-			// Safari
-			addEventListener('pagehide', () => status = 'hidden');
-			// Else
-			doc.addEventListener('visibilitychange', () => {
-				status = doc.visibilityState;
-				doc.hidden || wakeUp();
-			});
-			wakeUp();
-			['mousemove','keyup','touchstart'].forEach(t => doc.addEventListener(t, wakeUp));
-			['scroll','pageshow'].forEach(t => addEventListener(t, wakeUp));
-		};
-
-	this.ifvisible = {
-		idle: callback => {
-			init();
-			addEventListener(idle, callback);
-		},
-		now: () => {
-			init();
-			return status === visible;
-		}
-	};
-})(document);