QT: Remove border around widgets

src/citra_qt/main.ui

@@ -24,7 +24,20 @@
    <bool>true</bool>
   </property>
   <widget class="QWidget" name="centralwidget">
-   <layout class="QHBoxLayout" name="horizontalLayout"/>
+   <layout class="QHBoxLayout" name="horizontalLayout">
+    <property name="leftMargin">
+     <number>0</number>
+    </property>
+    <property name="topMargin">
+     <number>0</number>
+    </property>
+    <property name="rightMargin">
+     <number>0</number>
+    </property>
+    <property name="bottomMargin">
+     <number>0</number>
+    </property>
+   </layout>
   </widget>
   <widget class="QMenuBar" name="menubar">
    <property name="geometry">