Remove the xingmux plugin, as the element has moved into mpegaudioparse in -ugly.

Remove the xingmux plugin, as the element has moved into mpegaudioparse in -ugly.

Original commit message from CVS:
* configure.ac:
* docs/plugins/gst-plugins-bad-plugins-docs.sgml:
* docs/plugins/gst-plugins-bad-plugins-sections.txt:
* docs/plugins/inspect/plugin-xingheader.xml:
* gst/xingheader/Makefile.am:
* gst/xingheader/gstxingmux.c:
* gst/xingheader/gstxingmux.h:
* tests/check/Makefile.am:
* tests/check/elements/.cvsignore:
* tests/check/elements/xingmux.c:
* tests/check/elements/xingmux_testdata.h:
Remove the xingmux plugin, as the element has moved into
mpegaudioparse in -ugly.