Revert "DSound: use DSound notifications to produce sound."

This reverts commit 4990b8910bc0fe5d576f92396f761a79b3f1a7e3.

The commit is causing substantial performance issues for the DSound
backend which I somehow didn't catch during testing.
Merge pull request #440 from magumagu/dsound-revert

Revert "DSound: use DSound notifications to produce sound."