remove unused button in categorized renderer ui

resources/customization.xml

@@ -391,7 +391,6 @@
     <widget objectName="btnAddCategory" class="QPushButton" label="Add"/>
     <widget objectName="btnDeleteCategories" class="QPushButton" label="Delete"/>
     <widget objectName="btnDeleteAllCategories" class="QPushButton" label="Delete all"/>
-    <widget objectName="btnJoinCategories" class="QPushButton" label="Join"/>
     <widget objectName="btnAdvanced" class="QPushButton" label="Advanced"/>
   </widget>
   <widget objectName="QgsTileScaleWidget" class="QWidget" label="">

src/ui/qgscategorizedsymbolrendererv2widget.ui

@@ -176,16 +176,6 @@
        </property>
       </spacer>
      </item>
-     <item>
-      <widget class="QPushButton" name="btnJoinCategories">
-       <property name="enabled">
-        <bool>false</bool>
-       </property>
-       <property name="text">
-        <string>Join</string>
-       </property>
-      </widget>
-     </item>
      <item>
       <widget class="QPushButton" name="btnAdvanced">
        <property name="text">
@@ -219,7 +209,6 @@
   <tabstop>btnAddCategory</tabstop>
   <tabstop>btnDeleteCategories</tabstop>
   <tabstop>btnDeleteAllCategories</tabstop>
-  <tabstop>btnJoinCategories</tabstop>
   <tabstop>btnAdvanced</tabstop>
  </tabstops>
  <resources/>