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

Makefile.am

@@ -828,7 +828,6 @@ libdecoder_a_CPPFLAGS = $(AM_CPPFLAGS) \
 	$(WAVPACK_CFLAGS) \
 	$(MAD_CFLAGS) \
 	$(MPG123_CFLAGS) \
-	$(MP4V2_CFLAGS) \
 	$(OPUS_CFLAGS) \
 	$(FFMPEG_CFLAGS) \
 	$(MPCDEC_CFLAGS) \
@@ -848,7 +847,6 @@ DECODER_LIBS = \
 	$(WAVPACK_LIBS) \
 	$(MAD_LIBS) \
 	$(MPG123_LIBS) \
-	$(MP4V2_LIBS) \
 	$(OPUS_LIBS) \
 	$(FFMPEG_LIBS2) \
 	$(MPCDEC_LIBS) \
@@ -963,12 +961,6 @@ noinst_LIBRARIES += libmodplug_decoder_plugin.a
 DECODER_LIBS += libmodplug_decoder_plugin.a $(MODPLUG_LIBS)
 endif
 
-if HAVE_MP4V2
-libdecoder_a_SOURCES += \
-	src/decoder/plugins/Mp4v2DecoderPlugin.cxx \
-	src/decoder/plugins/Mp4v2DecoderPlugin.hxx
-endif
-
 if ENABLE_SIDPLAY
 libdecoder_a_SOURCES += \
 	src/decoder/plugins/SidplayDecoderPlugin.cxx \