decoder/mp4v2: remove because of incompatible license

libmp4v2 is licensed under MPL 1.1, which is incompatible with GPLv2.
Unfortunately, this means that we must remove the plugin.

More information can be found in the Debian bug report:

 http://bugs.debian.org/767504

configure.ac

@@ -496,11 +496,6 @@ AC_ARG_ENABLE(modplug,
 		[enable modplug decoder plugin]),,
 	enable_modplug=auto)
 
-AC_ARG_ENABLE(mp4v2,
-	AS_HELP_STRING([--enable-mp4v2],
-		[enable libmp4v2 decoder plugin]),,
-	enable_mp4v2=auto)
-
 AC_ARG_ENABLE(mpc,
 	AS_HELP_STRING([--enable-mpc],
 		[disable musepack (MPC) support (default: auto)]),,
@@ -1264,24 +1259,6 @@ if test x$enable_modplug = xyes; then
 fi
 AM_CONDITIONAL(HAVE_MODPLUG, test x$enable_modplug = xyes)
 
-dnl -------------------------------- libmp4v2 ---------------------------------
-if test x$enable_aac = xyes; then
-        MPD_AUTO_LIB(mp4v2, MP4V2, mp4v2, MP4Create, [-lmp4v2], [],
-	                    [mp4v2], [libmp4v2 not found])
-
-        if test x$enable_mp4v2 = xyes; then
-	        AC_DEFINE(HAVE_MP4V2, 1, [Define to use libmp4v2 for MP4 decoding])
-        fi
-else
-     if test x$enable_mp4v2 = xyes; then
-        AC_MSG_ERROR([MP4V2 requires AAC!])
-     fi
-
-     enable_mp4v2=no
-fi
-
-AM_CONDITIONAL(HAVE_MP4V2, test x$enable_mp4v2 = xyes)
-
 dnl -------------------------------- libopus ----------------------------------
 MPD_AUTO_PKG(opus, OPUS, [opus ogg],
 	[opus decoder plugin], [libopus not found])
@@ -1868,7 +1845,6 @@ printf '\n\t'
 results(sndfile, [libsndfile])
 results(mikmod, [MikMod])
 results(modplug, [MODPLUG])
-results(mp4v2, [MP4V2])
 results(mad, [MAD])
 results(mpg123, [MPG123])
 results(mpc, [Musepack])