welcome screen: fix borders for OS != Windows

src/plugins/coreplugin/welcomemode.ui

@@ -162,10 +162,10 @@ background-color: qlineargradient(spread:pad, x1:0.5, y1:0, x2:0.5, y2:1, stop:0
 }</string>
            </property>
            <property name="frameShape">
-            <enum>QFrame::StyledPanel</enum>
+            <enum>QFrame::NoFrame</enum>
            </property>
            <property name="frameShadow">
-            <enum>QFrame::Raised</enum>
+            <enum>QFrame::Plain</enum>
            </property>
            <layout class="QGridLayout" name="gridLayout_2">
             <property name="leftMargin">