Remove build flags that no longer exist

The 'mimic' codec was used by MSN messenger in prehistoric times, and
was removed [1]. The 'nas' and 'xvid' plugins were removed because
they were never ported to GStreamer 1.0 [2].

The 'mpg123' plugin was moved from -bad to -ugly [3].

No need to issue a new build just for this.

[1] gst-plugins-bad commit ad661999ad2c79a5
    https://bugzilla.gnome.org/show_bug.cgi?id=597616

[2] gst-plugins-bad commit 9b5de053995488d5
    https://bugzilla.gnome.org/show_bug.cgi?id=774530

[3] gst-plugins-bad commit 08d8aefcdaaf89ec
    https://bugzilla.gnome.org/show_bug.cgi?id=719849

gstreamer1-plugins-bad-free.spec

@@ -210,10 +210,10 @@ autoreconf --force --install
     --with-package-origin="http://download.fedoraproject.org" \
     %{!?with_extras:--disable-fbdev --disable-decklink --disable-linsys} \
     --enable-debug --disable-static --enable-gtk-doc --enable-experimental \
-    --disable-dts --disable-faac --disable-faad --disable-nas \
-    --disable-mimic --disable-libmms --disable-mpeg2enc --disable-mplex \
-    --disable-neon --disable-rtmp --disable-xvid \
-    --disable-flite --disable-mpg123 --disable-sbc --disable-opencv \
+    --disable-dts --disable-faac --disable-faad \
+    --disable-libmms --disable-mpeg2enc --disable-mplex \
+    --disable-neon --disable-rtmp \
+    --disable-flite --disable-sbc --disable-opencv \
     --disable-spandsp --disable-voamrwbenc --disable-x265
 
 %make_build