wasapisink: Don't call CoUninitialize() twice in unprepare()

It has to be symmetric with CoInitialize(), otherwise everything else
will fail.
7 jobs for wasapisink-coinitialize in 26 minutes and 58 seconds (queued for 1 second)
Status Job ID Name Coverage
  Preparation
passed #72303
gst indent

00:00:32

passed #72302
manifest

00:00:32

 
  Build
passed #72304
build fedora x86_64

00:02:05

 
  Test
passed #72338
check fedora

00:16:55

failed #72305
check fedora

00:19:06

 
  Full Builds
passed #72306
build android arm64 api28

00:03:55

passed #72307
build cerbero fedora x86_64

00:07:21