fix Twang.cpp

Was producing noise (values far in excess of +/- 1.0) This is with both VC++12 and VC++11, using the eguitar project.
r̶e̶v̶e̶r̶t̶e̶d̶ ̶t̶h̶e̶ ̶l̶i̶n̶e̶ ̶r̶e̶m̶o̶v̶e̶d̶ ̶s̶o̶m̶e̶t̶i̶m̶e̶ ̶s̶i̶n̶c̶e̶ ̶4̶.̶4̶.̶4̶
nope, my mistake, can't find that line anywhere, perhaps I added it myself ages ago and forgot.