[mobile] Remove bannerimage from drawer

We don't make use of banner images in most other apps.

It also appears to be broken anyway.

mobile/CMakeLists.txt

 add_subdirectory(plugins)
 
-qt5_add_resources(KALGEBRAMOBILE_SRCS resources.qrc header.qrc)
+qt5_add_resources(KALGEBRAMOBILE_SRCS resources.qrc)
 add_executable(kalgebramobile pluginsmodel.cpp
     ../src/consolemodel.cpp
     clipboard.cpp

mobile/header.qrc

-<RCC>
-    <qresource>
-        <file>header.jpg</file>
-    </qresource>
-</RCC>

mobile/plugins/widgets/kde/KAlgebraMobile.qml

@@ -37,7 +37,6 @@ Kirigami.ApplicationWindow
 
         title: "KAlgebra"
         titleIcon: "qrc:/kalgebra.svgz"
-        bannerImageSource: "qrc:/header.jpg"
         drawerOpen: true
 
         Instantiator {