drivers: staging: rtl8712: _adapter is declared twice

struct _adapter has been declared at 23rd line.
Remove the duplicate.

Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Link: https://lore.kernel.org/r/20210325080050.861273-1-wanjiabing@vivo.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rtl8712/drv_types.h

@@ -36,8 +36,6 @@ enum _NIC_VERSION {
 	RTL8716_NIC
 };
 
-struct _adapter;
-
 struct	qos_priv	{
 	/* bit mask option: u-apsd, s-apsd, ts, block ack... */
 	unsigned int qos_option;