Remove beam ending rules in 6/8, 9/8 and 12/8 time

Remove beam ending rules in 6/8, 9/8 and 12/8 time

 - Now that beatGrouping works correctly, beam
   ending rules are not required for those values
   of beatGrouping that are set up by default in
   scm/music-functions.scm.  At present these are
   set just for 6/8, 9/8 and 12/8 signatures.

 - The beam ending rules for 1/32 notes are retained,
   however, but are extended to cover those moments
   which previously were covered by the deleted rules.