fix #24245: slurs in voice > 1 attach to wrong voice on load

The end track was being set based on the start track at a time when the start track had not actually been read, so it kept defaulting to voice 0.  The code to copy the start track to end track *after* the read was already present, so I just removed the premature copy and all seems well.  Tested to work for single-voice slurs (voice > 1) as well as cross voice slurs, both in 2.0 and 1.3 scores.