Spatializer: remove EffectClient nested class

Remove unused nested class EffectClient. It was designed
to help expose an IEffect interface allowing avanced configuration
by spatializer implementor but will be replaced by the addition
of a set/get parameter methods on ISpatializer.

Bug: 188502620
Test: make
Change-Id: Ia3b4740f1b00cb444b6e4e7e039765421ef0297b