gnu: cairo: Remove duplicate input.

This is a follow-up to bb8be806b5.

* gnu/packages/gtk.scm (cairo)[inputs]: Remove the extra FONTCONFIG.  While at
it, reintroduce source code comments that disappeared with the same commit.

gnu/packages/gtk.scm

@@ -162,8 +162,8 @@ (define-public cairo
         ;; "--enable-cogl=yes"
         ;; "--enable-directfb=yes"
         ;; "--enable-vg=yes"
-        "--enable-tee=yes"
-        "--enable-xml=yes"
+        "--enable-tee=yes"              ;needed for GNU IceCat
+        "--enable-xml=yes"              ;for cairo-xml support
         (string-append "--with-html-dir="
                        (assoc-ref %outputs "doc")
                        "/share/gtk-doc/html"))))
@@ -180,7 +180,6 @@ (define-public cairo
      `( ;; ("cogl" ,cogl)
        ;; ("directfb" ,directfb)
        ("fontconfig" ,fontconfig)
-       ("fontconfig" ,fontconfig)
        ("freetype" ,freetype)
        ("glib" ,glib)
        ;; ("gtk+" ,gtk+)