ecore_audio: remove pure virtual functions from classes

Summary:
A call to a undefined function results in a NOP we can get the same
behaviour with @empty. In Ecore.Audio this pure_virtual function makes
sense, so make the class abstract.

ref T7632

Reviewers: cedric, segfaultxavi, zmike, q66

Reviewed By: segfaultxavi

Subscribers: reviewers, committers

Tags: efl

Maniphest Tasks: T7632

Differential Revision: https://phab.enlightenment.org/D7669