Reducing trim at low bitrate

Some informal tests seem to confirm that reducing the trim at 32-64 kbps
improves quality (better HF). It's not clear whether it's also the case
at 96 kb/s and above, so we're leaving it as is for those rates.

This corresponds to buildC in this thread:
https://hydrogenaud.io/index.php/topic,113985.0.html

Also see:
https://hydrogenaud.io/index.php/topic,111798.0.html