Remove Xfce-specific display manager tip

This is already covered in the previous chapter https://github.com/NixOS/nixpkgs/blob/master/nixos/doc/manual/configuration/x-windows.xml#L38-L46 - Xfce also depends on X and it doesn't need a special section on display managers.

Changed files
+1 -12
nixos
doc
manual
configuration
+1 -12
nixos/doc/manual/configuration/xfce.xml
···
To install them manually (system wide), put them into your
<literal>environment.systemPackages</literal>.
</para>
-
-
<para>
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NixOS’s default <emphasis>display manager</emphasis> is SLiM.
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(DM is the program that provides a graphical login prompt
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and manages the X server.)
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You can, for example, select KDE’s
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<command>sddm</command> instead:
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<programlisting>
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services.xserver.displayManager.sddm.enable = true;
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</programlisting>
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</para>
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+
<simplesect>
<title>Thunar Volume Support</title>