delivery: drop the dead audio import placeholder in voicesink (V-581)

audio.PCM16kMono was only referenced by a `var _ =` placeholder whose
comment claimed to keep the import "honest" for a method call that
doesn't need it — out.Format's IsValid() is a method value, calling it
never requires importing the package that defines the type. The import
had no other use in the file, so both it and the placeholder were dead.

internal/delivery/voicesink/voicesink.go

@@ -34,7 +34,6 @@ import (
 	"log"
 	"time"
 
-	"github.com/kami/maven/internal/audio"
 	"github.com/kami/maven/internal/delivery"
 	"github.com/kami/maven/internal/tts"
 	"github.com/kami/maven/internal/ttsnorm"
@@ -102,9 +101,3 @@ func (s *Sink) Send(ctx context.Context, send delivery.Sendable) error {
 	}
 	return nil
 }
-
-// keep audio import honest (used in Send's audio.PCM check indirect via
-// Format.IsValid which is a method on the imported audio.Format). The alias
-// below keeps the import alive even if a future refactor moves the only
-// reference. Today, the synthesizer's audio.Audio directly flows through.
-var _ = audio.PCM16kMono