playlist/lastfm: remove defunct Last.fm support

This plugin has been defunct ever since Last.fm changed their
protocol.  Since there is no volunteer willing to fix the plugin, I'm
removing it now.

src/PlaylistRegistry.cxx

@@ -23,7 +23,6 @@
 #include "playlist/ExtM3uPlaylistPlugin.hxx"
 #include "playlist/M3uPlaylistPlugin.hxx"
 #include "playlist/XspfPlaylistPlugin.hxx"
-#include "playlist/LastFMPlaylistPlugin.hxx"
 #include "playlist/DespotifyPlaylistPlugin.hxx"
 #include "playlist/SoundCloudPlaylistPlugin.hxx"
 #include "playlist/PlsPlaylistPlugin.hxx"
@@ -53,9 +52,6 @@ const struct playlist_plugin *const playlist_plugins[] = {
 #ifdef ENABLE_DESPOTIFY
 	&despotify_playlist_plugin,
 #endif
-#ifdef ENABLE_LASTFM
-	&lastfm_playlist_plugin,
-#endif
 #ifdef ENABLE_SOUNDCLOUD
 	&soundcloud_playlist_plugin,
 #endif