Fix beatGroups in 5/4, 6/4, 7/4, 9/4, 12/4

[vanderstel]

This PR removes a code block that was causing 5/4, 6/4, 7/4, 9/4, and 12/4 to create triple beat subdivisions:

Previous:

const ts = new music21.meter.TimeSignature('6/4');
console.log(ts.computeBeatGroups());
>>> [[3, 8], [3, 8], [3, 8], [3, 8]]

Current:

const ts = new music21.meter.TimeSignature('6/4');
console.log(ts.computeBeatGroups());
>>> [[2, 8]]

[mscuthbert]

This is great -- I will be moving the rest of music21p meter code here, but it will take some time. This is a much better interim step.