Note filters: remove unnecessary calculations in Formantfilter

Formantfilter, being the heaviest of the utility filters in LMMS, has the most to gain from optimization. First, I'd like to chop off some unnecessary code: there are two LPFs being calculated and not used for anything.

I've checked and tested the change using this: https://github.com/softrabbit/filterbench and measured the speed benefit to be up to 25%. (32-bit Linux, compiled with `-msse2 -mfpmath=sse -ffast-math`)