theme: update system grey background

Fixes https://gitlab.gnome.org/GNOME/gnome-shell/issues/2174

data/gnome-shell-theme.gresource.xml

@@ -18,7 +18,6 @@
     <file alias="icons/message-indicator-symbolic.svg">message-indicator-symbolic.svg</file>
     <file>no-events.svg</file>
     <file>no-notifications.svg</file>
-    <file>noise-texture.png</file>
     <file>pad-osd.css</file>
     <file alias="icons/eye-open-negative-filled-symbolic.svg">eye-open-negative-filled-symbolic.svg</file>
     <file alias="icons/eye-not-looking-symbolic.svg">eye-not-looking-symbolic.svg</file>

data/theme/gnome-shell-sass/widgets/_screen-shield.scss

@@ -70,8 +70,7 @@ $_screenshield_shadow: 0px 0px 6px rgba(0, 0, 0, 0.726);
 }
 
 #lockDialogGroup {
-  background: lighten(#2e3436, 8%) url(resource:///org/gnome/shell/theme/noise-texture.png);
-  background-repeat: repeat;
+  background-color: lighten(#2e3436, 8%);
 }
 
 #screenShieldNotifications {

js/ui/background.js

@@ -504,12 +504,10 @@ var SystemBackground = GObject.registerClass({
     Signals: { 'loaded': {} },
 }, class SystemBackground extends Meta.BackgroundActor {
     _init() {
-        let file = Gio.File.new_for_uri('resource:///org/gnome/shell/theme/noise-texture.png');
 
         if (_systemBackground == null) {
             _systemBackground = new Meta.Background({ meta_display: global.display });
             _systemBackground.set_color(DEFAULT_BACKGROUND_COLOR);
-            _systemBackground.set_file(file, GDesktopEnums.BackgroundStyle.WALLPAPER);
         }
 
         super._init({