decoder/mad: remove option "gapless", always do gapless

Why would anybody want to keep the silence inserted by the codec?
Other plugins/codecs (such as Vorbis) have this hard-coded as well.

src/decoder/plugins/MadDecoderPlugin.cxx

@@ -62,12 +62,8 @@ enum class MadDecoderMuteFrame {
 /* the number of samples of silence the decoder inserts at start */
 static constexpr unsigned DECODERDELAY = 529;
 
-static constexpr bool DEFAULT_GAPLESS_MP3_PLAYBACK = true;
-
 static constexpr Domain mad_domain("mad");
 
-static bool gapless_playback;
-
 gcc_const
 static SongTime
 ToSongTime(mad_timer_t t) noexcept
@@ -100,14 +96,6 @@ mad_fixed_to_24_buffer(int32_t *dest, const struct mad_pcm &src,
 			*dest++ = mad_fixed_to_24_sample(src.samples[c][i]);
 }
 
-static bool
-mad_plugin_init(const ConfigBlock &block)
-{
-	gapless_playback = block.GetBlockValue("gapless",
-					       DEFAULT_GAPLESS_MP3_PLAYBACK);
-	return true;
-}
-
 class MadDecoder {
 	static constexpr size_t READ_BUFFER_SIZE = 40960;
 
@@ -735,7 +723,7 @@ MadDecoder::DecodeFirstFrame(Tag *tag) noexcept
 
 		struct lame lame;
 		if (parse_lame(&lame, &ptr, &bitlen)) {
-			if (gapless_playback && input_stream.IsSeekable()) {
+			if (input_stream.IsSeekable()) {
 				/* libmad inserts 529 samples of
 				   silence at the beginning and
 				   removes those 529 samples at the
@@ -1030,6 +1018,5 @@ static const char *const mad_mime_types[] = { "audio/mpeg", nullptr };
 
 constexpr DecoderPlugin mad_decoder_plugin =
 	DecoderPlugin("mad", mad_decode, mad_decoder_scan_stream)
-	.WithInit(mad_plugin_init)
 	.WithSuffixes(mad_suffixes)
 	.WithMimeTypes(mad_mime_types);