nixpkgs/doc: remove unused `unfree.xml`

This section was broken out into its own file in efb55d2a42 but
apparently never actually got included anywhere.

Since then a more detailed section on using unfree packages has been
introduced.

Copied the introduction and removed the rest of the file.

doc/builders/packages/unfree.xml

@@ -1,13 +0,0 @@
-<section xmlns="http://docbook.org/ns/docbook"
-         xmlns:xlink="http://www.w3.org/1999/xlink"
-         xml:id="unfree-software">
- <title>Unfree software</title>
-
- <para>
-  All users of Nixpkgs are free software users, and many users (and developers) of Nixpkgs want to limit and tightly control their exposure to unfree software. At the same time, many users need (or want) to run some specific pieces of proprietary software. Nixpkgs includes some expressions for unfree software packages. By default unfree software cannot be installed and doesn’t show up in searches. To allow installing unfree software in a single Nix invocation one can export <literal>NIXPKGS_ALLOW_UNFREE=1</literal>. For a persistent solution, users can set <literal>allowUnfree</literal> in the Nixpkgs configuration.
- </para>
-
- <para>
-  Fine-grained control is possible by defining <literal>allowUnfreePredicate</literal> function in config; it takes the <literal>mkDerivation</literal> parameter attrset and returns <literal>true</literal> for unfree packages that should be allowed.
- </para>
-</section>

doc/using/configuration.chapter.md

@@ -77,6 +77,11 @@ The difference between a package being unsupported on some system and being brok
 
 ## Installing unfree packages {#sec-allow-unfree}
 
+All users of Nixpkgs are free software users, and many users (and developers) of Nixpkgs want to limit and tightly control their exposure to unfree software.
+At the same time, many users need (or want) to run some specific pieces of proprietary software.
+Nixpkgs includes some expressions for unfree software packages.
+By default unfree software cannot be installed and doesn’t show up in searches.
+
 There are several ways to tweak how Nix handles a package which has been marked as unfree.
 
 -   To temporarily allow all unfree packages, you can use an environment variable for a single invocation of the nix tools: