By SCOTT NISHIMURA Bell Helicopter laid off 30 Tarrant County workers who received their walking papers a week ago and left the company Friday."In response to a reduction in demand for commercial aircraft, we had to adjust our work force and cut 42 hourly positions in our production facilities," said Thomas Dolney, a Bell spokesman."Twelve of the affected employees were placed into other positions within the company, so a total of 30 people were laid off today.