Revert "pcm: file: Add htimestamp operation in plugin file"

This reverts commit f8d07a73.

The htimestamp field was already intiailized.  It was just overlooked,
sorry.

Suggested-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>