check.h: remove obsolete header

Since we switched from autotools to Meson in commit
94592c1406, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

test/test_translate_song.cxx

@@ -2,7 +2,6 @@
  * Unit tests for playlist_check_translate_song().
  */
 
-#include "config.h"
 #include "playlist/PlaylistSong.hxx"
 #include "song/DetachedSong.hxx"
 #include "SongLoader.hxx"