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/config/Migrate.cxx

@@ -55,17 +55,8 @@ MigrateCurlProxyConfig(ConfigData &config) noexcept
 				 "proxy_password");
 }
 
-static void
-MigrateMadConfig(ConfigData &config) noexcept
-{
-	MigrateParamToBlockParam(config, ConfigOption::GAPLESS_MP3_PLAYBACK,
-				 ConfigBlockOption::DECODER, "plugin", "mad",
-				 "gapless");
-}
-
 void
 Migrate(ConfigData &config) noexcept
 {
 	MigrateCurlProxyConfig(config);
-	MigrateMadConfig(config);
 }