Fix Tab key cycle through slide background panel

Remove empty widget container, lot of garbage placeholders and
redundant adjustment.

Regression from commit fba28df77d7e98663d1cf77ac206f8799fce4108
Oh dear, how could I possibly have reviewed this in?

Change-Id: I25af642afad094faba5b0ba1e37cfbf1a954e52f
Reviewed-on: https://gerrit.libreoffice.org/47720
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>
(cherry picked from commit aa957dffaa40a7d4f34ec90ea50779532bdc9540)
Reviewed-on: https://gerrit.libreoffice.org/47840
Reviewed-by: Yousuf Philips <philipz85@hotmail.com>
diff --git a/sd/uiconfig/simpress/ui/sidebarslidebackground.ui b/sd/uiconfig/simpress/ui/sidebarslidebackground.ui
index 5578327..868741e 100644
--- a/sd/uiconfig/simpress/ui/sidebarslidebackground.ui
+++ b/sd/uiconfig/simpress/ui/sidebarslidebackground.ui
@@ -3,11 +3,6 @@
<interface domain="sd">
  <requires lib="gtk+" version="3.0"/>
  <requires lib="LibreOffice" version="1.0"/>
  <object class="GtkAdjustment" id="adjustment1">
    <property name="upper">100</property>
    <property name="step_increment">1</property>
    <property name="page_increment">10</property>
  </object>
  <object class="GtkGrid" id="SlideBackgroundPanel">
    <property name="visible">True</property>
    <property name="can_focus">False</property>
@@ -278,63 +273,10 @@
          </object>
          <packing>
            <property name="left_attach">1</property>
            <property name="top_attach">16</property>
            <property name="top_attach">12</property>
          </packing>
        </child>
        <child>
          <object class="GtkBox" id="container">
            <property name="visible">True</property>
            <property name="can_focus">False</property>
            <property name="orientation">vertical</property>
            <child>
              <placeholder/>
            </child>
          </object>
          <packing>
            <property name="left_attach">2</property>
            <property name="top_attach">0</property>
          </packing>
        </child>
        <child>
          <placeholder/>
        </child>
        <child>
          <placeholder/>
        </child>
        <child>
          <placeholder/>
        </child>
        <child>
          <placeholder/>
        </child>
        <child>
          <placeholder/>
        </child>
        <child>
          <placeholder/>
        </child>
        <child>
          <placeholder/>
        </child>
        <child>
          <placeholder/>
        </child>
        <child>
          <placeholder/>
        </child>
        <child>
          <placeholder/>
        </child>
        <child>
          <placeholder/>
        </child>
        <child>
          <placeholder/>
        </child>
        <child>
          <placeholder/>
        </child>
        <child>
          <placeholder/>
        </child>
        <child>