ob-lilypond: Code cleanup

ob-lilypond: Code cleanup

* lisp/ob-lilypond.el (org-babel-lilypond-OSX-ly-path,
  org-babel-lilypond-OSX-pdf-path, org-babel-lilypond-OSX-midi-path,
  org-babel-lilypond-nix-ly-path, org-babel-lilypond-nix-pdf-path,
  org-babel-lilypond-nix-midi-path, org-babel-lilypond-w32-ly-path,
  org-babel-lilypond-w32-pdf-path, org-babel-lilypond-w32-midi-path,
  org-babel-lilypond-determine-ly-path,
  org-babel-lilypond-determine-pdf-path,
  org-babel-lilypond-determine-midi-path): Remove.
  (org-babel-lilypond-ly-command, org-babel-lilypond-midi-command,
  org-babel-lilypond-pdf-command): Replacement for removed variables
  and functions.  Adapt all calls to the removed functions to use
  these variables instead.
  (org-babel-lilypond-commands): New defcustom for setting up
  the *-command variables.  Keep different defaults for different
  systems as the original code did to avoid tripping up unsuspecting
  users.
  (org-babel-lilypond-execute-tangled-ly,
  org-babel-lilypond-check-for-compile-error): Revert conditions to
  avoid superfluous forms.  Remove unused return values.
* testing/lisp/test-ob-lilypond.el: Do test for new variables and
  replace removed function calls with the appropriate variable
  content.  Exercise the new defcustom thoroughly.