buildGoPackage: remove Go deps file libs.json

After #16017 there were a lot
of comments saying that `nix` would be better than `JSON`
for Go packages dependency sets.
As said in https://github.com/NixOS/nixpkgs/pull/16017#issuecomment-229624046

    > Because of the content-addressable store, if two programs have the
    > same dependency it will already result in the same derivation in
    > the
    > store. Git also has compression in the pack files so it won't make
    > much difference to duplicate the dependencies on disk. And finally
    > most users will just use the binary builds so it won't make any
    > differences to them.

This PR removes `libs.json` file and puts all package dependencies in
theirs `deps.json`.

doc/languages-frameworks/go.xml

@@ -86,13 +86,6 @@ the following arguments are of special significance to the function:
           "rev": "a83829b6f1293c91addabc89d0571c246397bbf4",
           "sha256": "1m4dsmk90sbi17571h6pld44zxz7jc4lrnl4f27dpd1l8g5xvjhh"
         }
-    },
-    {
-    "include": "../../libs.json", <co xml:id='ex-goDeps-4' />
-    "packages": [ <co xml:id='ex-goDeps-5' />
-            "github.com/docopt/docopt-go",
-            "golang.org/x/crypto",
-        ]
     }
 ]
 </programlisting>
@@ -122,20 +115,6 @@ the following arguments are of special significance to the function:
     </para>
   </callout>
 
-  <callout arearefs='ex-goDeps-4'>
-    <para>
-      <varname>include</varname> could be used to reuse <varname>goDeps</varname> between Go programs.
-      There is a common libs set in <varname>&lt;nixpkgs/pkgs/development/go-modules/libs.json&gt;</varname>
-      with pinned versions of many packages that you can reuse.
-    </para>
-  </callout>
-
-  <callout arearefs='ex-goDeps-5'>
-    <para>
-      <varname>packages</varname> enumerates all Go packages that will be imported from included file.
-    </para>
-  </callout>
-
 </calloutlist>
 
 </para>