images: Remove .jp-FolderIcon already in icon

The icon.css already has a .jp-FolderIcon which sets the background url
to landingfolder.svg. For some reason the class and svg file are
duplicated in images.css and default-theme/images. This is unnecessary
duplication - so, remove it from images and use icons instead.

src/default-theme/images.css

@@ -47,8 +47,3 @@
 .jp-Landing-tour {
   background-image: url(images/tour.svg);
 }
-
-
-.jp-FolderIcon {
-  background-image: url(images/landingfolder.svg);
-}

src/default-theme/images/landingfolder.svg

@@ -1,21 +0,0 @@
-<?xml version="1.0" encoding="UTF-8" standalone="no"?>
-<svg width="14px" height="14px" viewBox="0 0 10 9" version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
-    <!-- Generator: Sketch 39 (31667) - http://www.bohemiancoding.com/sketch -->
-    <title>ic_folder_black_24px</title>
-    <desc>Created with Sketch.</desc>
-    <defs></defs>
-    <g id="Page-1" stroke="none" stroke-width="1" fill="none" fill-rule="evenodd">
-        <g id="Artboard-1" transform="translate(-664.000000, -578.000000)">
-            <g id="Launcher" transform="translate(655.000000, 291.000000)">
-                <g id="Group-3" transform="translate(8.000000, 284.851852)">
-                    <g id="ic_folder_black_24px" transform="translate(0.000000, 1.000000)">
-                        <g id="Group">
-                            <path d="M5,2 L2,2 C1.45,2 1.005,2.45 1.005,3 L1,9 C1,9.55 1.45,10 2,10 L10,10 C10.55,10 11,9.55 11,9 L11,4 C11,3.45 10.55,3 10,3 L6,3 L5,2 L5,2 Z" id="Shape" fill="#9E9E9E"></path>
-                            <polygon id="Shape" points="0 0 12 0 12 12 0 12"></polygon>
-                        </g>
-                    </g>
-                </g>
-            </g>
-        </g>
-    </g>
-</svg>