Replace 32bit buffer olapBufferSynth2_fx with 16bit buffer olapBufferSynth2

Bug description

For IVAS, in addition to the existing 16 bit buffer olapBufferSynth2 for FD-CNG, a 32 bit buffer was introduced, and both need to be maintained in parallel.

It should be checked, whether the 32 bit buffer could be removed again.