The Tennessee city that calls itself Music City U.S.A. just lost its major source of classical music--the Nashville Symphony orchestra. The ensemble's board of directors has announced it will lay off the symphony's 85-member orchestra and conductor Kenneth Schermerhorn because of long-standing financial problems. Orchestra representatives tried to assemble the orchestra to perform a scheduled concert Friday night but failed. Dennis Bottorff, chairman of the orchestra's board, said $900,000 would be needed to complete the season. Lewis Lavine, who heads a task force appointed last week to review the symphony's options, said disbanding the orchestra was the only alternative left after labor talks were broken off last week.