module-equalizer-sink: use fftwf_free in all the right places

Several mallocs were made using fftwf_malloc and were erroneously freed by
pa_xfree. This patch corrects these calls of pa_xfree to fftwf_free.
module-equalizer-sink: remove erroneous fixme

pa_memblockq_drop is guaranteed to drop the chunk we just extracted from the
memblockq which internally unrefs the chunk.