After Xfce 4.20 release remove outdated "backdrop" settings

The "backdrop" background picture handling was extensively reworked in
Xfce 4.20 [1]. Unfortunately, the default backdrop image gets set when
compiling the package [2]. User-specific changes are stored in local config
file "xfce4-desktop.xml" by property "last-image" per workspace. However,
workspaces are assigned to monitors and they have hardware-specific names.
Deriving local monitor preferences from generic ones worked before but it
doesn't seem to work anymore.

Summarizing, currently there seems no way to set the postmarketOS wallpaper
as the default backdrop image.

[1] https://gitlab.xfce.org/xfce/xfdesktop/-/commit/edb653c37038fcfce61c424ce841c4f5aa0faaf1
[2] https://gitlab.xfce.org/xfce/xfdesktop/-/blob/xfdesktop-4.20.0/meson.build#L112

config/xfce4/xfconf/xfce-perchannel-xml/xfce4-desktop.xml

 <?xml version="1.0" encoding="UTF-8"?>
 
 <channel name="xfce4-desktop" version="1.0">
-  <property name="backdrop" type="empty">
-    <property name="screen0" type="empty">
-      <property name="monitor0" type="empty">
-        <property name="image-path" type="string" value="/usr/share/wallpapers/postmarketos.jpg"/>
-        <property name="image-style" type="int" value="5"/>
-        <property name="image-show" type="bool" value="true"/>
-      </property>
-      <property name="monitor1" type="empty">
-        <property name="image-path" type="string" value="/usr/share/wallpapers/postmarketos.jpg"/>
-        <property name="image-style" type="int" value="5"/>
-        <property name="image-show" type="bool" value="true"/>
-      </property>
-      <property name="monitor2" type="empty">
-        <property name="image-path" type="string" value="/usr/share/wallpapers/postmarketos.jpg"/>
-        <property name="image-style" type="int" value="5"/>
-        <property name="image-show" type="bool" value="true"/>
-      </property>
-      <property name="monitor3" type="empty">
-        <property name="image-path" type="string" value="/usr/share/wallpapers/postmarketos.jpg"/>
-        <property name="image-style" type="int" value="5"/>
-        <property name="image-show" type="bool" value="true"/>
-      </property>
-    </property>
-  </property>
   <property name="desktop-icons" type="empty">
     <property name="single-click" type="bool" value="true"/>
     <property name="file-icons" type="empty">