gnucash: delete icon theme cache

pkgs/applications/office/gnucash/default.nix

···
         --set GCONF_CONFIG_SOURCE 'xml::~/.gconf'                       \
         --prefix PATH ":" "$out/bin:${perl}/bin:${gconf}/bin"
     done
+
+    rm $out/share/icons/hicolor/icon-theme.cache
   '';
 
   # The following settings fix failures in the test suite. It's not required otherwise.