Remove unused audio mk files

These are empty and useless.

Change-Id: I23cd1be105f7bddbf555a35e10efcc92a7c1eb47

Patchset: 1
http://review.cyanogenmod.org/#/c/56508/
(cherry picked from commit 618c8dd0e424cfa3cc267ec0b6c1a232ef80d17a)