audio: delete unused ril-client API.

The clock sync func is unused for both HSPA and LTE device.
Picked from : https://github.com/CyanogenMod/android_device_samsung_tuna/commit/5d52b3587788685e81f1d96c0abe778ab94b5b6e#L1R172

Change-Id: I638ace774c318c0bf7b51606bb909112e40ae376