lilypond-book: Remove snippet map

Support was broken ever since commit 38e1cad4 ("Fix the linking of files from lilypond by lilypond-book.") prepended the current directory as relative path to the list of includes in 2012. This made LilyPond find the snippets as ./??/lily-*.ly and the whole mapping did not work. Now that commit 06cdcec7 ("lilypond-book: Convert to absolute paths earlier") uses absolute paths and the --lily-output-dir is not in the list of include paths, the mapping came back to life and it becomes apparent why nobody missed it:

• It's redundant since --use-source-file-names will replace the input file name with the original source file name.
• Since the snippet map is added as a regular entry to the list of input files, it is subject to processing with multiple jobs. In particular this means:
  • without the argument -djob-count or a requested number of jobs greater than the number of input files (which also results in no jobs being forked), things might work;
  • if jobs are forked (and be it only one), the list of input files gets reversed and the map comes last after all files have been processed; and
  • even if that was fixed, only one job gets to see the map.

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Contains one commit to honor verbosity in lilypond-book to always print the invoked command, and a second to remove the the defunct filename mapping support in LilyPond itself.