Remove unused debugging switches

The preprocessor macros DEBUG_SLUR_SCORING, DEBUG_TIE_SCORING and DEBUG_BEAM_SCORING in lily/include/main.hh were used to toggle several code paths that allow for showing scoring information for slurs, ties and beams.

These paths are active by default for a very long time now (~ 15 years). The preprocessor checks are superfluous, since the functionality has to be explicitly activated with the layout variables debug-{slur,tie,beam}-scoring, anyway.

@hanwenn As the original author of these checks, please say if I'm missing something here.