Drop unused IsSystemFont config property

Unused since 2014:
https://cgit.freedesktop.org/libreoffice/core/commit/?id=324de04a408a77656a473cbfd2cc1ce77664b0c0
and
https://cgit.freedesktop.org/libreoffice/core/commit/?id=6c415f735bbf9201a18288e2ea16eacad4d635cd

Change-Id: I06f7a6510312307d3ab4a94c45e7f882630c8408
Reviewed-on: https://gerrit.libreoffice.org/41126Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Andras Timar <andras.timar@collabora.com>

officecfg/registry/schema/org/openoffice/Office/Common.xcs

@@ -6080,14 +6080,6 @@
         </info>
         <value>false</value>
       </prop>
-      <prop oor:name="IsSystemFont" oor:type="xs:boolean" oor:nillable="false">
-        <info>
-          <desc>Allows the user to change the UI font to the system
-          default.</desc>
-          <label>Use system default user interface font</label>
-        </info>
-        <value>true</value>
-      </prop>
       <prop oor:name="IsSelectionInReadonly" oor:type="xs:boolean" oor:nillable="false">
         <info>
           <desc>Indicates whether the cursor is displayed in read-only