Revert "ALSA: usb-audio: Disable Lenovo P620 Rear line-in volume control"

This reverts commit 34dedd2a.

According to Realtek, volume FU works for line-in.

I can confirm volume control works after device firmware is updated.

Fixes: 34dedd2a ("ALSA: usb-audio: Disable Lenovo P620 Rear line-in volume control")
Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Link: https://lore.kernel.org/r/20200915103925.12777-1-kai.heng.feng@canonical.comSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/mixer_maps.c

@@ -371,7 +371,6 @@ static const struct usbmix_name_map asus_rog_map[] = {
 };
 
 static const struct usbmix_name_map lenovo_p620_rear_map[] = {
-	{ 19, NULL, 2 }, /* FU, Volume */
 	{ 19, NULL, 12 }, /* FU, Input Gain Pad */
 	{}
 };